Fixed visual artifacts and running voxelized Sponza model

I fixed the visual artifacts which were due to a numerical error in the code.  Here's a screenshot of my engine running a voxelized version of the Sponza model commonly used to test graphics engines and GPUs.  The model contains 9,002,961 non-air voxels.  I'm right up against my 60 frames per second frame-rate cap, so it's running well!  It will look much better once I get flood fill lighting implemented.