message: Set transmission roughness to zero in UsdPreviewSurface
diff: <input name="weight" type="float" value="1" />
  <input name="tint" type="color3" value="1, 1, 1" />
  <input name="ior" type="float" interfacename="ior" />
  + <input name="roughness" type="vector2" value="0.0, 0.0" />
  <input name="normal" type="vector3" nodename="surface_normal" />
  <input name="scatter_mode" type="string" value="T" />
  </dielectric_bsdf>
  