Double-sided distribution sampling #26
Description
Currently the surface distributions support a cosine product sampling optimization but it can be applied only once per distribution and it can't be undone.
Since the distribution initialization is quite heavy it could be beneficial to initialize it once and then sample from it multiple times.
For materials with multiple layers (e.g. diffuse and refractive transmission) it could be efficient to sample both sides of the surface (one side might not need the cosine product optimization) but the current surface distribution API doesn't seem to allow that.