message: Improvements to gltf_pbr translation graph
  Leave glTF PBR emissive and emissive_strength at their defaults when Standard Surface emission is zero.
  Leave glTF PBR sheen_roughness at its default when Standard Surface sheen is zero.
diff: <output name="clearcoat_out" type="float" />
  <output name="clearcoat_roughness_out" type="float" />
  <output name="emissive_out" type="color3" />
  - <output name="emissive_strength_out" type="float" />
  </nodedef>
  <nodegraph name="NG_standard_surface_to_gltf_pbr" nodedef="ND_standard_surface_to_gltf_pbr">
  <input name="in1" type="color3" interfacename="sheen_color" />
  <input name="in2" type="float" interfacename="sheen" />
  </multiply>
  - <dot name="sheen_roughness" type="float">
  - <input name="in" type="float" interfacename="sheen_roughness" />
  - </dot>
  + <ifgreater name="sheen_roughness" type="float">
  + <input name="value1" type="float" interfacename="sheen" />
  + <input name="value2" type="float" value="0" />
  + <input name="in1" type="float" interfacename="sheen_roughness" />
  + <input name="in2" type="float" value="0" />
  + </ifgreater>
  <!-- Clearcoat -->
  <ifequal name="clearcoat" type="float">
  </dot>
  <!-- Emission -->
  - <dot name="emissive" type="color3">
  - <input name="in" type="color3" interfacename="emission_color" />
  - </dot>
  - <dot name="emissive_strength" type="float">
  - <input name="in" type="float" interfacename="emission" />
  - </dot>
  + <multiply name="emissive" type="color3">
  + <input name="in1" type="color3" interfacename="emission_color" />
  + <input name="in2" type="float" interfacename="emission" />
  + </multiply>
  <output name="base_color_out" type="color3" nodename="base_color" />
  <output name="metallic_out" type="float" nodename="metallic" />
  <output name="clearcoat_out" type="float" nodename="clearcoat" />
  <output name="clearcoat_roughness_out" type="float" nodename="clearcoat_roughness" />
  <output name="emissive_out" type="color3" nodename="emissive" />
  - <output name="emissive_strength_out" type="float" nodename="emissive_strength" />
  </nodegraph>
  </materialx>
  