Slightly Better

Uniform grid acceleration in all 3 dimension now.  Better/faster raymarching, better grid storage management, even some rudimentary compression.

Still pretty underwhelming results, but at least I can get >10K tris on screen with this method.  That's way better than before.  I'm still nowhere near PBRT, and CPU + MLT is looking pretty good right now...but I just can't let go of the hope that, if I persevere with this GPU path tracer, I may get lucky in the end :)