Generator: Shape initial refactoring.

The code compiles, but several structure generators are broken, crash on start.
Gen refactor: Implemented CompositedHeiGen.

This fixes crashes in the Village generator due to the missing generator.
DungeonRooms: Changed to work with the new shape generators.
ChunkDesc: Fixed comment about indexing.
Snow generator: Fixed failure at top of the world.
Generator: Fixed crash with trees too high.
Mobs: Fixed crash with terrain too high.
Merge remote-tracking branch 'origin/master' into GeneratorShapeRefactor
Noise3D generators: Changed noise generator to InterpolNoise.
Generators: Unified SeaLevel into a single variable.

It is shared between shape generators and composition generators and there's no sense in having two different values for those.
CompoGenBiomal: Fixed sealevel not generating properly.
CompoGenBiomal: Fixed sealevel offset.
BiomalNoise3D generator: finished all biomes.
cWorld: Changed generator defaults.
cWorld: Rewritten spawn preparation.

It now supports pregeneration distance of any size and runs in two threads in parallel (generator / lighting).
Fixes #1597.
Merge remote-tracking branch 'origin/master' into GeneratorShapeRefactor
Noise3D generator: Enlarged averaging to avoid steep beach slopes.
Added TwoHeights shape generator.

This is a faster shape generator that can generate overhangs and has biome awareness.
Changed back capitalization.
Merge pull request #1611 from mc-server/SpawnPrepare

cWorld: Rewritten spawn preparation.