Add tileCount uniform

Allows changing the tile coordinate calculation to use a custom `tileCount` (number of tiles per row in texture atlas) instead of hardcoded 16.0.

Note: not backwards-compatible (can uniforms be made optional?), changes for voxel-mipmap-demo to set the uniform here: https://github.com/deathcap/voxel-mipmap-demo/commits/tilecount