Change textureIndex calculation, no effect for 512x512 atlasSize. Ref GH-4
Detect texture atlas overflow

Ref https://github.com/deathcap/voxel-texture-shader/issues/2
Break out textureIndex, factor in tilePad into tileCount
Working with 256x256 atlas size ref GH-4 with ao-shader fsh tileCount

Requires deathcap/ao-shader@68ddaa770cadc1c37d0b0a3be66dae60e65f296f