Process voxel array into solidVoxels, applying opaque bit from registry

Instead of being baked into the voxel array directly, the
opaque/transparent bit is now added in on blocks with the 'transparent'
property set in voxel-registry. This hides the opaque bit implementation
detail from the voxel format, so for example, voxel-engine getBlock()
and setBlock() on glass is now e.g., 46, instead of 32814 -- the
opaqueness is internal to the meshing engine, not exposed outside.

Ref GH-5
0.12.0