Smooth the underside of overhangs and ledges too.
Terrain pieces exposed to air on top use the grass and side textures.

BUG: Cubes should use the side texture sometimes too.
BUG: It might be a good idea to implement post-processing for textures to
     allow more complicated rules for setting textures. As an example of
	 where this is useful, look at ramps which sit on top of cubes.
Cubes use the SIDE textures when appropriate.
Terrain post-processor no longer limited to four sets of rules.
Terrain post-processor allocates less memory and uses one less memcpy.
Terrain generation policy moved from GSChunkStore to GSTerrain.