message: Treat PBRT substrate material like plastic
diff: @@ -84,6 +84,18 @@ std::shared_ptr<sg::material> make_material_node(Shape::SP shape)
  return obj;
  }
  + else if (auto m = std::dynamic_pointer_cast<SubstrateMaterial>(mat))
  + {
  + auto obj = std::make_shared<sg::obj_material>();
  +
  + obj->cd = vec3(m->kd.x, m->kd.y, m->kd.z);
  + obj->cs = vec3(m->ks.x, m->ks.y, m->ks.z);
  +
  + // TODO
  + obj->specular_exp = m->uRoughness;
  +
  + return obj;
  + }
  else if (auto m = std::dynamic_pointer_cast<MirrorMaterial>(mat))
  {
  }
  