Face culling for custom block models

[deathcap]

Cull adjacent faces (if allowed by the model, as specified in block-models cullFace: true) on custom blocks (2nd shader) if needed. This will reduce the number of vertices and might fix:

(using water and lava from voxel/voxel-fluid@30a4456)

[mikolalysenko]

This is a bug in the mesher. It should eliminate/merge those facets.