message: added clarification in MultiBSDF
diff: @@ -121,7 +121,8 @@ inline BSDF_SampleRes MultiBSDF_sample(const varying BSDF* uniform super,
  if (eq(res.weight, make_vec3f(0.0f)) | (res.pdf == 0.0f))
  return make_BSDF_SampleRes_zero();
  - // XXX why not also eval smooth BSDF when specular one was sampled?
  + // if the sampled BSDF is specular (has a delta distribution), we cannot add the contributions from the other BSDFs
  + // [Pharr et al., 2016, "Physically Based Rendering", 3rd Edition, Section 14.1.6]
  if (res.type & BSDF_SPECULAR)
  {
  // scale by rcp(selection pdf)
  