Add support for per-side voxel texturing

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 https://github.com/deathcap/voxel-stitch/commit/c05021b5efe433da8a935f8a8c58d049021315e8