message: Fix crash in MultiBSDF
  invalid BSDFs could still be chosen in the rare case of random number == 0.0f
diff: @@ -106,7 +106,8 @@ inline BSDF_SampleRes MultiBSDF_sample(const varying BSDF* uniform super,
  float x = ss * self->importanceSum;
  int choice = 0;
  float prefixSum = self->importances[0];
  - while ((choice < self->numBsdfs-1) & (x > prefixSum))
  + // to handle ss==0.0 and invalid BSDF (importance==0.0), use "x >="
  + while ((choice < self->numBsdfs-1) & (x >= prefixSum))
  prefixSum += self->importances[++choice];
  // remap sample
  