Modified this example to use reduce_equal() to see if all of the program
instances want to load the 8 sample values around the same voxel. When
this is the case, we can just do 8 scalar loads, rather than needing to
do a fully general gather. Once this check fails, it isn't done again,
since it's not likely to start succeeding in the future. This gives
a ~10% speedup with the low-res data set, and basically no performance
difference with the high-res one. (It makes sense that the lower-resolution
the voxel sampling, the longer all of the rays will stay in the same set
of voxels.)
