Use transferrable type ArrayBuffer in webworker post message to avoid copy
Use game.arrayType for chunk voxels instead of hardcoding Uint8Array
Allow tree generation to cross chunks (decoration vs population)

Previously chunks were 'populated' with trees, editing the chunk voxel
data directly, before it was sent to the engine for rendering, but this
limited feature generation. Add a new 'decorate' phase and message, where
voxel-land generates a list of world global coordinates and values, then
sends them to voxel-engine using setBlock() (may need to change later if
is too slow) -- this allows the generated features to span multiple chunks,
as long as they are loaded. Use new treetype:2 space-filling tree as a demo.
Fix tree generation surface level
Variable tree density and type
Add craters (proto-lakes)
Fix floating trees generating on top of craters

Partially. Not complete fix since decoration at extend outside loaded
chunks, and then setBlock is ignored.