Adds a birch-like tree to the default_game. Aspen was chosen on
purpose instead of birch, as several birch tree mods already exist
and choosing a different name avoids any conflicts.

Schematics were made for both normal and sapling version, assuring
saplings will not be able to grief. The shape of the Aspen is "fanning
out at the top" and provides an easy tree to walk under, but still a
somewhat thick cover. The Aspen trunk is 4 to 6 blocks tall, with up
to three layers of leaves on top, making it slightly taller than an
Apple tree, but shorter than a Pine tree, which provides a good blend.

Textures were painted from scratch, except tree_top and _wood
variants, which are color modified versions of the pine_wood
variants. Appropriate licenses have been chosen for the new textures.

The leaf texture is light enough to contrast other trees, but dark
enough to offset against our light default grass color. The leaves
are drawn in the typical minetest default fashion with plenty of
transparancy, but enough definition to suggest that you're seeing
something that looks like leaves. The placement of leaves in the
schematic also suggests the top of the tree is sparse and you can
see the sky through the leaves.

Sapling texture is both traditional and different, with lush green
leaves and a well-defined stem, but slightly stick-like and skinny,
as these plants tend to grow up first, then out.

Add fallen Aspen logs. We make these logs a minimum of 2 blocks long,
and up to 3. This allows us to make these logs a place where both
red and brown mushrooms can be found, to these may be attractive to
players. However, the spawn rate for these has been reduced a lot
compared to the other logs, to account for the scarcity of Aspen.

Add stairs, slabs for these wood types as well.

Mapgen will place these trees in deciduous forests only, but in
a way that the biome is a range between entirely Apple trees, and
mostly entirely Aspen trees, with a bias to Apple trees. To make
fallen logs somewhat correlated with trees, we modify the planting
of Apple trees and logs to use perlin noise and not fill ratio,
otherwise you'd always end up with Apple logs in Aspen tree areas,
which would be suspicious. There still is a bit of a mix.

In oversight, I added this recipe not verifying that it was already
taken.

We change this to a 2x2 iron bar recipe. The shape and amount are
reasonable (reduced to output 1 steel trapdoor), and I verified that
it wasn't in use.

Fixes #779

These were likely too high, so we lower them to the rate
of mushrooms on apple trees.

issue #773

This is a non-issue: these are aliased to normal mushrooms. For
clarity, we replace them with the normal mushrooms here as well.

We can vary the landscape a bit more by placing "fallen logs"
around the various forests. These decorations are quite fast
and will provide some gameplay value but are still more rare
than the corresponding trees, so they don't provide free
materials.

I've manually put the schematic as lua tables since these log
schematics are only 8 blocks. We vary the log lengths between
1 and 3 blocks by making the end blocks have a lower chance
of appearing.

Amount is varied by fill_ratio, except for acacia trees where
we reduce the scale, so that acacia logs show up near places with
acacia trees consistently.

Mushrooms are placed optionally on each log. We can't place
two different mushrooms on a log, so instead we opt to place
brown mushrooms on oak/appletree logs, brown mushrooms on
jungletree logs, and red mushrooms on pine logs. No mushrooms
are placed on acacia logs, as they occur in a dry biome,
savannah, and this adds a bit of biome diversity.

Closes #770 - thanks to @tenplus1, @kilbith

Combine any written book with an empty book to copy it. The
copy is in player hands when using, and the original is put
back on the crafting grid and can be directly copied again.

All ownership and metadata is retained, so the copy of the book
is as writable as the original is, or isn't.

Adds a steel trapdoor. Textures were painted from scratch, and
inspired by the current Steel Door. Ownership on the trapdoor
works as expected, and so does the crafting recipe.

Playing stereo sounds positionally in OpenAL causes it to play
the sounds unattenuated - same volume for all distances. This
shouldn't happen, and makes door sounds unneccesarily loud from
very far away.

Convert all door samples to mono, 22kHz 64kbit.

Trimmed all door samples to remove lead, trail silence.

These have almost no value gameplay wise. Mushrooms spread
by their very nature into appropriate soil nearby, and
harvesting spores is something only scientists do. Actual
mushroom farmers leave a few mushrooms and put on more
manure, and keep the light off.

Modify the growth ABM to test for light levels and attempt
to plant a similar mushroom nearby (+/- 2 x,z, +/- y). Light
at both source and destination needs to be low enough. I've
tuned it to be 3-4 spaces from a torch.

Mushrooms will die in full sunlight, but they can survive under
trees and may grow out at night anywhere.

Removed obsolete textures.

Remove unused nodes and provide aliases for them.

Aliased obsolete nodes so no unknown item nodes appear.

Mushrooms die only in full sunlight (light level 15).

This prevents nodes that are not of the "facedir" paramtype2,
as these are generally not expected to be able to rotate. Mods
and other methods may still manipulate param2 of these nodes
as expected.

Addresses #712

Re-align parameter format in 'register ore'

Since written books are quite different from empty books, the
visual clue that they are different items is really needed in-game.

I've added a "clasp" or "belt-with-buckle" like element to the
png from default_book, keeping them very similar but also
immediately recgnizably different.

I added the new texture to blockmen's license list since it's
obviously derivative of his work.

The PNG was run through a minimizer/optimizer to save space.

To make water a little clearer and feel purer
Also correct lava alpha values from 192 to 191

The fixing of low density decorations allows returning to
sidelen 16 for acacia trees and cacti, previously sidelen
80 was required for low density decorations to appear
Also use sidelen 16 instead of 8 for mgv6 papyrus

To avoid processing spikes where catch-up is non-essential
Disable in: Lavacooling, grass growing, grass removal
and all fire mod ABMs

Bring humidity points closer to reduce distortion of voronoi cells
Slightly reduce size of hot and frozen biomes
Improve location of glacier biomes
Remove unnecessary snow nodeboxes from tundra to improve FPS
Add missing dirt_with_snow to taiga

Update 'disable fire' documentation in conf.example

Closes #544