Create voxelSideTextureIDs [voxel,side]->textureID array

For deathcap/ao-mesher@bfc352f8210d9538956ed658527e0810cd70a1e3
Add expandName
Enhance expandName for 5-element array differing front/back; throw on invalid length>6 sides
Assign per-texture voxel indices for each voxel type
Fix textureNames indexOf in array check
Skip falsey texture side names (no texture); fixes demo