Issue #811 - new gravel texture needed.

This texture was Gambits' PixelBOX gravel light texture. Gambit
posted that his texture pack is WTFPL:
- https://forum.minetest.net/viewtopic.php?f=4&t=4990&start=50#p141196

I've made significant modifications to this texture:
- slightly rotated and rolled some sections of pixels
- minor burn/dodge some pixels to keep high contrast
- removed lineair repeating effects
- etc.

Attribution is added back to Gambit. Thanks.

Both the standing and sitting animations had misplaced curve
cusps that caused the end part of the animation to wiggle the
feet slightly back and forward.

I've fixed both animations parts and re-exported. Verified in-game
with multiplayer that everything was indeed fixed.

Spread ABM intervals evenly across 1 to 16 seconds
16s ensures no nodes are missed when player walks past
Adjust chance values to compensate, for identical action rates
Combine lavacooling ABMs into one, return to chance = 1
Grass growth: add 'neighbors = "air"' to avoid
processing the thousands of underground dirt nodes
Grass death: Reduce action rate to that of grass growth
Fire: Use chance = 1 for flame extinguishing
and flame removal when mod is disabled

Node timers are higher precision and a better guarantee
of happening at regular intervals, whereas ABM's may be
postponed, cancelled or missed if a player is too far.

The largest benefit is that once the furnace is done
cooking, no more ABM's are fired - the timer is stopped
instead and no more events are created until items
are put in the furnace.

This patch is larger due to the migration of the timer
function and indentation change as a result of the somewhat
reduced complexity. I've tested with several furnaces and
this works correctly and behavior is not affected, although
people may find that their furnaces now work more
regularly.

If you place several furnaces next to eachother, you will
still find all furnace timers firing exactly at the same
time. This is a bug in core that should not coalesce node
timers at second intervals.

Avoids trees or large cacti being placed multiple times at a single
position, by changing the node at the heightmap y value

This converts the call to minetest.register() for the default
fence node, so it can be called by other mods to quickly
setup other fences.

Since this creates an API, insert it into the game_api.txt.

The api looks like minetest.register(name, {def}), and has two
uncommon fields: "texture" and "material". Any normal nodedef
property can be passed through, except "drawtype". The "fence"
group will always be added.

The default fence recipe is modified to be as follows:
  wood, stick, wood
  wood, stick, wood

This recipe yields 4 fence nodes.

This allows us to create according recipes for acacia, pine,
aspen, and junglewood fences without adding new stick types:

  pine wood, stick, pine wood
  pine wood, stick, pine wood

This is a from-scratch implementation, written by heart but inspired
by (#665 - Add many wooden fences).

Stick and fences nodes are named in a consistent way.

We may want to use biomes here at a later stage.

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.

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

issue #773

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.

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.

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

Closes #544

Dirt waters are more suitable for waterlilies
Add dedicated dunes biome in coniferous forest
Papyrus: use dirt base again
Force-place roots on mapgen tree schematics
Tune some humidity points to improve voronoi cells
Add random rotation to jungle trees

New brighter mineral_mese texture