Rotate front and left facing UVs -90 degrees

Currently in voxel-texture I am [rotating the front and left textures 90 degrees](https://github.com/shama/voxel-texture/blob/master/index.js#L45) when loading a texture onto a mesh to compensate for the UVs.

I think it would be more efficient to rotate the UVs on the mesh instead. This change will return UVs rotated -90 degrees for the front and left faces. Thanks!