This commit condenses the above methods into a single implementation used by
V7, V5, Flat, Fractal, and Valleys mapgens and introduces MapgenBasic.

BiomeGen defines an interface that, given a set of BiomeParams, computes biomes
for a given area using the algorithm implemented by that specific BiomeGen.
This abstracts away the old system where each mapgen supplied the noises
required for biome generation.

We can remove the function in MtNativeActivity now
as it serves precisely that purpose: to tell irrlicht
that we handled the esc key.

TODO for later:
 * Perhaps try to find a more performant container than KeyList

Broken by b1965ac2.
This also prepares the begin and commit statements only once.

Fixes a bug where packet reordering made the server give the
client two peer ids instead of one. This in turn confused
reliable packet sending and made connecting to the server fail.

The client usually sends three packets at init: one "dummy"
packet consisting of two 0 bytes, and the init packet as well as
its legacy counterpart. The last one can be turned off since commit
af301831, but this is of lower
relevance for the bug. The relevant part here is that network
packet reorder (which is a normal occurence) can make the packets
reach the server in different order.

If reorder puts the dummy packet further behind, the following
would happen before the patch:

1. The server will get one of the init packets on channel 1 and
   assign the client a peer id, as the packet will have zero as
   peer id.

2. The server sends a CONTROLTYPE_SET_PEER_ID packet to inform
   the client of the peer id.

3. The next packet from the client will contain the peer id set by
   the server.

4. The server sets the m_has_sent_with_id member for the client's
   peer structure to true.

5. Now the dummy packet arrives. It has a peer id of zero, therefore
   the server searches whether it already has a peer id for the
   address the packet was sent from. The search fails because
   m_has_sent_with_id was set to true and the server only searched
   for peers with m_has_sent_with_id set to false.

6. In a working setup, the server would assign the dummy packet to
   the correct peer id. However the server instead now assigns a
   second peer id and peer structure to the peer, and assign the
   packet to that new peer.

7. In order to inform the peer of its peer id, the server sends a
   CONTROLTYPE_SET_PEER_ID command packet, reliably, to the peer.
   This packet uses the new peer id.

8. The client sends an ack to that packet, not with the new peer id
   but with the peer id sent in 2.

9. This packet reaches the server, but it drops the ACK as the peer
   id does not map to any un-ACK-ed packets with that seqnum. The
   same time, the server still waits for an ACK with the new peer
   id, which of course won't come. This causes the server to
   periodically re-try sending that packet, and the client ACKing it
   each time.

Steps 7-9 cause annoyances and erroneous output, but don't cause
the connection failure itself.
The actual mistake that causes the connection failure happens in 6:
The server does not assign the dummy packet to the correct peer, but
to a newly created one.
Therefore, all further packets sent by the client on channel 0 are
now buffered by the server as it waits for the dummy packet to reach
the peer, which of course doesn't happen as the server assigned
that packet to the second peer it created for the client.
This makes the connection code indefinitely buffer the
TOSERVER_CLIENT_READY packet, not passing it to higher level code,
which stalls the continuation of the further init process
indefinitely and causes the actual bug.

Maybe this can be caused by reordered init packets as well, the only
studied case was where network has reliably reordered the dummy
packet to get sent after the init packets.

The patch fixes the bug by not ignoring peers where
m_has_sent_with_id has been set anymore. The other changes of the
patch are just cleanups of unused methods and fields and additional
explanatory comments.

One could think of alternate ways to fix the bug:

* The client could simply take the new peer id and continue
  communicating with that. This is however worse than the fix as
  it requires the peer id set command to be sent reliably (which
  currently happens, but it cant be changed anymore). Also, such a
  change would require both server and client to be patched in order
  for the bug to be fixed, as right now the client ignores peer id
  set commands after the peer id is different from
  PEER_ID_INEXISTENT and the server requires modification too to
  change the peer id internally.
  And, most importantly, right now we guarantee higher level server
  code that the peer id for a certain peer does not change. This
  guarantee would have to be broken, and it would require much
  larger changes to the server than this patch means.

* One could stop sending the dummy packet. One may be unsure whether
  this is a good idea, as the meaning of the dummy packet is not
  known (it might be there for something important), and as it is
  possible that the init packets may cause this problem as well
  (although it may be possible too that they can't cause this).

Thanks to @auouYmous who had originally reported this bug and who
has helped patiently in finding its cause.

Version 1.8.2 of irrlicht changed the way that IGUIStaticText::getTextHeight() works and since that release properly deals with newlines.

From irrlicht changes.txt for 1.8.2, "IGUIStaticText::getTextHeight returns now the correct height for texts with newlines even WordWrap is not set."

* Remove the copy from db::loadBlock by using a pointer to the destination
* cleanup db backend, the child backend doesn't have to set their functions as virtual

This fixes an issue where trees are omitted from Mapgen V6 maps on
configurations that explicitly defined the mgv6_spflags setting.

Doing it the other way round was a mistake, since it breaks
minetest.formspec_escape with escape sequences that contain
special characters.

This makes it work even if it contains escape sequences,
which didn't work before.

Fixes the issue introduced by c1a0ebb7 (Fix use of uninitialised variable
in class Event) causing Windows builds to fail

The path finding code works fairly well except that it considers
anythin not CONTENT_AIR to be "above the surface". This results in
paths that are unwalkable for entities since e.g. plants are not
walkable. The path would force them to jump on top of grass plants,
etc..

The obvious solution is not to use CONTENT_AIR as a criteria, but
instead distinguish between walkable and non-walkable nodes. This
results in paths that properly walk through grass nodes.

This was extensively tested by a flock of electric sheep.

Note that for underwater purposes this changes the behaviour from
"the surface is walkable" to "ignore water entirely" making the
path go across the water bottom, and pathing fail likely from the
water surface. This is intentional.

Before, the GridNodes were stored in vector<vector<vector<T>>>,
and initialized in advance. Putting three vectors inside each other
puts lots of unneccessary stress onto the allocator, costs more memory,
and has worse cache locality than a flat vector<T>.

For larger search distances, an the array getting initialized means
essentially O(distance^3) complexity in both time and memory,
which makes the current path search a joke. In order to really
profit from the dijkstra/A* algorithms, other data structures
need to be used for larger distances.

For shorter distances, a map based GridNode storage may be slow as
it requires lots of levels of indirection, which is bad for things like
cache locality, and an array based storage may be faster.

This commit does:

1. remove the vector<vector<vector<T>>> based GridNodes storage that
	is allocated and initialized in advance and for the whole
	possible area.

2. Add a vector<T> based GridNodes storage that is allocated and
	initialized in advance for the whole possible area.

3. Add a map<P,T> based GridNodes storage whose elements are
	allocated and initialized, when the path search code
	demands it.

4. Add code to decide between approach 2 and 3,
	based on the length of the path.

5. Remove the unused "surfaces" member of the PathGridnode class.
	Setting this isn't as easy anymore for the
	map based GridNodes storage.

There is no need to reinvent the wheel here, we have
great classes from irrlicht.

* Fix naming style for methods and classes:
	Use camelCase for methods and PascalCase for classes as
	code style demands it. And use sneak_case for methods that
	are not member of a class.
* Replace "* " with " *" for Pointers
* Same for references
* Put function body opening braces on new line
* Other misc minor non functional style improvements

There is no need to put them into the header, they are solely used
inside the pathfinder.

Another advantage of this change is that only the pathfinder.cpp has
to be compiled if PATHFINDER_DEBUG gets defined or undefined, not
all files including the .h.

This commit moves the pathfinder classes to the cpp file without
modifications.
Also, the PATHFINDER_DEBUG macro gets moved to the cpp file and
the PATHFINDER_CALC_TIME macro gets moved to a plce where it
actually does work.

I broke this in 46fd114e.

Correct parameter names mg_valleys to mgvalleys
Remove biome NoiseParams from MapgenValleysParams
Improve format of parameter code