message: Add support for transmission_extra_roughness in Standard Surface
  This change list adds support for parameter transmission_extra_roughness, that was previously unused in the MaterialX standard_surface implementation.
diff: <nodegraph name="NG_standard_surface_surfaceshader_100">
  <!-- Roughness influence by coat-->
  + <!-- Calculate main specular roughness -->
  <multiply name="coat_affect_roughness_multiply1" type="float">
  <input name="in1" type="float" interfacename="coat_affect_roughness" />
  <input name="in2" type="float" interfacename="coat" />
  <input name="roughness" type="float" nodename="coat_affected_roughness" />
  <input name="anisotropy" type="float" interfacename="specular_anisotropy" />
  </roughness_anisotropy>
  + <!-- Calculate transmission roughness -->
  + <add name="transmission_roughness_add" type="float">
  + <input name="in1" type="float" interfacename="specular_roughness" />
  + <input name="in2" type="float" interfacename="transmission_extra_roughness" />
  + </add>
  + <clamp name="transmission_roughness_clamped" type="float">
  + <input name="in" type="float" nodename="transmission_roughness_add" />
  + </clamp>
  + <mix name="coat_affected_transmission_roughness" type="float">
  + <input name="fg" type="float" value="1.0" />
  + <input name="bg" type="float" nodename="transmission_roughness_clamped" />
  + <input name="mix" type="float" nodename="coat_affect_roughness_multiply2" />
  + </mix>
  + <roughness_anisotropy name="transmission_roughness" type="vector2">
  + <input name="roughness" type="float" nodename="coat_affected_transmission_roughness" />
  + <input name="anisotropy" type="float" interfacename="specular_anisotropy" />
  + </roughness_anisotropy>
  <!-- Tangent rotation -->
  <multiply name="tangent_rotate_degree" type="float">
  <input name="weight" type="float" value="1.0" />
  <input name="tint" type="color3" interfacename="transmission_color" />
  <input name="ior" type="float" interfacename="specular_IOR" />
  - <input name="roughness" type="vector2" nodename="main_roughness" />
  + <input name="roughness" type="vector2" nodename="transmission_roughness" />
  <input name="normal" type="vector3" interfacename="normal" />
  <input name="tangent" type="vector3" nodename="main_tangent" />
  <input name="distribution" type="string" value="ggx" />
  