factor out terrain_check and block_exists from chunk_gen
factor out Mesh from Chunk
terrain: thin layer of grass on top of stone
add dirt blocks
terrain: dont generate stone above dirt
add keybinding to toggle glPolygonMode
fix shader indentation
add water blocks
terrain_gen: add more low frequency noise
render faces separately
render back faces in line mode
cull back faces ourselves
use clamp
remove LOD

Complicates mesh optimization. Might add it back later.