Clear roughness from back of TransmissionRoughnessTest

It was noticed that path tracers and rasterizers see different amounts of roughness on a clear volume, given that the path tracer will take into account roughness on the back of a transmissive surface, but a rasterizer will not.

We could consider adjusting the TransmissionRoughnessTest model to remove the roughness from the backs of the sample tiles, so all the roughness is on the fronts only. This might allow a more direct comparison between the different rendering strategies.

Start by locating the TransmissionRoughnessTest model and inspect the roughness on the backs and fronts of its sample tiles. Update the model so roughness is present only on the fronts, then verify that path-tracer and rasterizer comparisons can use the same roughness arrangement.

Written by the indexing model from the issue text.