Add support for per-side voxel texturing

[deathcap]

Currently, the texture index is extracted directly from the voxel ID (voxel&0xff), so all faces have the same texture. This PR adds an optional voxelSideTextureIDs 2d ndarray argument to computeMesh(), which can be used to assign different textures to each of the sides.

Using this in voxel/voxel-stitch@c05021b

[deathcap]

Hm, I'm not sure if the side calculation is correct, it seems side passed to voxelTexture is always either 0 or 3 (if flipped). Testing texturing each of the six faces differently, but only the right-facing face (side 3) has a different texture, all others use the side 0 index:

[deathcap]

Fixed =) d+(flip?3:0)

[mikolalysenko]

Thanks! Merged