Remove opaque bit hacks, deprecate getPackedBlockIndex()

The mesher now internally handles the opaque bit:
deathcap/voxel-mesher@0f9527240975c7c19aaf7163eb4ae56e7fef9f3a

getPackedBlockIndex() is now equivalent to getBlockIndex(); packed
indices are no longer used in the registry or high-level engine.

Ref https://github.com/deathcap/voxel-mesher/issues/5
0.7.0