Fix awkward countVoxelID calculation

1<<15 === 2<<14, not sure why I was using the latter earlier.
Change texture arrays, now backed by ndarray-hash instead (sparse)
Change texture array to 1-based, leaving 0 as air (unused)

Wastes one slot, but avoids the need to +1 or -1 when accessing block
indices (and consistency with the three.js-based voxel-engine)
0.7.0