3D Texture

The first major step of gas week has been accomplished!  The 3D texture density lookup works perfectly, as does the texture generation kernel.

I must say, I'm impressed with the speed!  I had serious concerns about the overhead of a 3D texture lookup in the participating media raymarcher.  However, the hit seems completely reasonable.  Of course, I haven't tried super-high-resolution fields yet, but I'm not sure that will be necessary, given that even this 8x8x8 grid looks good enough with linear interpolation.