Tidy up position numbers and code
Check for flora group aswell
Improve node light check for mushroom growth
Shrooms grow in 3 and below light

This changes how dirt blocks turn to dirt_with -grass, -dry_grass
or -snow.

Previously, dirt that was sunlit would turn to dirt_with_grass no
matter what, but this happened without any context, so you could
get green patches of dirt_with_grass in the middle of a savannah or
even desert.

Dirt no longer turns to covered dirt unless it's within 1 node from
another dirt_with_grass or dirt_with_dry_grass or dirt_with_snow.
This makes dirt_with_grass "growback" a lot slower, since it now only
happens on the edges, but it retains the context nicely now.

If there is any dirt with a grass or dry grass plant, or snow on top,
and enough light, we'll convert it sporadically to dirt_with_grass
or dirt_with_dry_grass or dirt_with_snow.

This allows us to plant grass of our choice in a large dirt patch,
or in a region where otherwise that type of grass is not present.

This used to be done by 2 abms, but I've combined them in to a single
ABM that is ordered to run with maximum efficiency, solving for the
most common outcome first before attempting more complex checks.

This is technically "dirt with grass" that's just under a snow
cover, so in darkness the grass on these nodes will also die,
turning it into dirt.

This doesn't convert dirt_with_snow under snow.

This will make sand and gravel blocks on top of TNT actually fly
in the air.

  https://youtu.be/4omndVZijLc

Let's just call it "terminal" velocity.

We add on_create() handlers for both burning TNT and burning
gunpowder. Because gunpowder will explode TNT in 1 second,
and not 4, we need to modify the 4 second timer after we
make the TNT burning. Other mods can now place burning TNT
that will by default explode after 4 seconds.

I spotted two places where under stress (many explosions) luajit would
end up passing nil to these functions. I'm not entirely sure how,
but it seems good form to guard against it, which does make it
more robust. After this patch, I'm not able to crash the server. With
many explosions, it may still lag significantly, but always returns
in the end.

We define the blast intensity as the square of the tnt_radius, divided
by the square of the distance to the explosion center, where distance
is limited to 1 at the lower end.

When destroying nodes, we calculate the intensity for each node, and
only destroy the nodes when the intensity is 1.0 or larger. To avoid
perfectly spherical explosions, we make sure to retain a randomness
factor of 20%. This will make explosion edges jagged and not smooth,
but not too much.

We pass the calculated intensity to on_blast() functions as well,
except we take the jitter here out and make sure it's always 1.0
or larger.

We apply a log scale to the size of the stacks ejected, so that
in larger explosions we are getting larger stacks. For normal r=3
explosions, this gives stack sizes ~6-7 or so, but for r=10 explosions
it could end up giving stacks of 25+.V

With the new high res timer, we can make TNT much better create chain
reactions. This does exactly that.

The drops list already has quantities, so let's just select the one
with the highest quantity from it, and use that as tile. Fallback
tile will therefore only be used if explosion happens in air. Oh well.

- Fix spacing errors around = and ..
- Indentation fixes
- Break up long lines

Currently the TNT mod uses a dummy vaule of 1 for this.

We add a dirt-like particle (drawn from scratch, uses some
colors from default_dirt's palette) to spawn many particles
that have collision enabled around the center of the blast.

This has the effect of obscuring the center of the blast, as
that is a painfully visible empty area when the explosion happens,
as there's only a little spark.

The dirt particles bounce around the walls and floor a bit,
and disappear rapidly, well before the smoke particles disappear.

This is a nice visual distraction that obscures the sudden
appearance of the gaping hole, and makes it a whole lot more
believable.

But not too much.

TNT is a bit underwhelming at the moment. We can make it a bit
more interesting by ejecting not just one or two itemstacks,
but a bunch of them. This code splits up the drops into
separate itemstacks that are 2-5 items together, which
results in generally roughly 10 itemstacks being ejected.

Since now we have multiple ejecta, it makes sense to tune
the ejecta velocities a bit better to get the appearance of
an actual explosion better. The items will not all start
with the same vertical velocity, since that would look
like fireworks. Instead we give them all a different vertical
speed.

If the node is special and has an on_blast() handler, we need
to call it instead of getting node drops manually. However, we
do want to know if drops should be added for the special nodes,
so we modify the on_blast() handler code to allow the nodedef
handlers to pass back itemstacks. This could be used by e.g.
the doors mod to drop door items after a blast.

Since this API is documented in lua_api.txt, a separate PR will
be incoming to update the on_blast() documentation.

Don't compose the shelf side texture. The texture isn't transparent,
and unlikely to be in any case.

Some people borrowed the creative code for their sub-games with an exclusive attribution to celeron55.

This is frustrating since I've largely rewritten, redesigned and carefully maintained this mod for the last months.

I expect to be credited.

Fixes #1005

This doesn't flood snow or ice blocks.

Currently any doors viewed from underwater will disappear but removing the line 'use_texture_alpha = true,' seems to fix this.  Thanks to Thomas-S for finding this glitch.

We properly checked top half already, just not the top half
target location.

Assure both checked positions are not unknown nodes.

Fixes #991

The boat model had over 1700 tris (!) before this redo. I've reduced
the amount of tris to ~150, which is very reasonable given that there
are almost 45 faces to this model.

I've also spent a good hour re-UV unwrapping the entire model which
has a huuuge impact on the boats' appearance. As much as possible,
the boat now looks like it's made out of actual blocks of wood,
and I've even attempted to make the grains connect around edges,
appear in the same pattern and spacing, and generally make it look
like it's just a nice crafted thing out of several pieces of wood.

I've had to tweak the rudder part to make the texture actually have
square texture pixels. I did that by varying the vertical position of
the botton of the rudder handle and the top of the rudder bottom, which
worked well. I also had to 'slice' the rear face of the boat to prevent
a strange texture tear, probably due to non-flat surface somewhere,
but I couldn't spot the issue there anymore after adding 2 extra edges.

This looks totally like a new boat now.

 - http://i.imgur.com/stiVzsa.jpg

It's possible to hit this when using an e.g. charcoal mod
where a tree is both a fuel and a source.

Added a recipe to craft a clay block back into 4 clay lumps.