K-D Trees on the GPU!

I finally got k-d trees running in my path tracer.  The speedup isn't as great as I would have hoped (in fact, it's not much different than a uniform grid..), but I haven't optimized anything yet.

I'm hopeful that, with optimization, I'll see better results.  On the bright side, the kd tree takes less pre-processing time than the uniform grid, and is overall a much smarter structure that requires less tweaking.

Let's hope optimization pulls through...this tracer is *right* on the edge of being usable.