decrease chunk size

To speed up meshing.
increase number of chunks to compensate for decreased chunk size
disable surface curvature

Until it works with greedy meshing.
meshing: iterate over faces in 2D slices
factor out Block.is_opaque and Map.index from block_exists
implement greedy meshing

See http://0fps.wordpress.com/2012/06/30/meshing-in-a-minecraft-game/

Rectangular regions of faces with the same blocktype are combined into a single
face.