Pack textures and use textureOffset

[matthewturk]

If we were able to guarantee that all of our textures were the same size (which we may be able to, with a small amount of wasted memory, using the existing kdtree) we could utilize textureOffset for our volume rendering.

Alternately, by jamming the octree data in as a single texture, we could pass an index array in and use that. This could build on the work @cphyc did on the octree volume rendering in software for yt.