CPU/GPU image differences in zero-day scenes

[mmp]

A few things are coming out differently, for unclear reasons. e.g. the pipes in frame52.pbrt.

CPU:

GPU:

[mmp]

Turns out that this boils down to the GPU path not using ray differentials for texture filtering.

The parameterization on that model is funky, leading to a very wide filter. On the CPU, at each point it ends up using the average of the entire texture, while on the GPU, it point samples the finest MIP level. The finest MIP level is mostly black, with some white regions, so the top of the MIP pyramid is dark grey...

So, either WAI, or more reason to get some texture filtering implemented in the GPU path...