message: Two-sided shading when sampling matte material
diff: @@ -48,7 +48,10 @@ inline spectrum<U> matte<T>::sample(
  Generator& gen
  ) const
  {
  - return from_rgb(shade_rec.tex_color) * diffuse_brdf_.sample_f(shade_rec.normal, shade_rec.view_dir, refl_dir, pdf, inter, gen);
  +#if 1 // two-sided
  + n = faceforward( n, shade_rec.view_dir, shade_rec.geometric_normal );
  +#endif
  + return from_rgb(shade_rec.tex_color) * brdf_.sample_f(n, shade_rec.view_dir, refl_dir, pdf, inter, gen);
  }
  template <typename T>
  