message: Fix issue with thin-film in standard_surface implementation
  This change list improves the implementation of standard_surface. The thin-film BSDF operator is now applied to both the dielectric specular BSDF and the metal BSDF, following the Standard Surface specification.
diff: <input name="mix" type="float" interfacename="transmission" />
  </mix>
  - <!-- Thin-film -->
  - <thin_film_bsdf name="thin_film_bsdf" type="BSDF">
  - <input name="thickness" type="float" interfacename="thin_film_thickness" />
  - <input name="ior" type="float" interfacename="thin_film_IOR" />
  - </thin_film_bsdf>
  -
  <!-- Specular Layer -->
  <dielectric_bsdf name="specular_bsdf" type="BSDF">
  <input name="weight" type="float" interfacename="specular" />
  <input name="top" type="BSDF" nodename="specular_bsdf" />
  <input name="base" type="BSDF" nodename="transmission_mix" />
  </layer>
  - <layer name="specular_layer_with_thin_film" type="BSDF">
  - <input name="top" type="BSDF" nodename="thin_film_bsdf" />
  - <input name="base" type="BSDF" nodename="specular_layer" />
  - </layer>
  <!-- Metal Layer -->
  <multiply name="metal_reflectivity" type="color3">
  </conductor_bsdf>
  <mix name="metalness_mix" type="BSDF">
  <input name="fg" type="BSDF" nodename="metal_bsdf" />
  - <input name="bg" type="BSDF" nodename="specular_layer_with_thin_film" />
  + <input name="bg" type="BSDF" nodename="specular_layer" />
  <input name="mix" type="float" interfacename="metalness" />
  </mix>
  + <!-- Thin-film Layer -->
  + <thin_film_bsdf name="thin_film_bsdf" type="BSDF">
  + <input name="thickness" type="float" interfacename="thin_film_thickness" />
  + <input name="ior" type="float" interfacename="thin_film_IOR" />
  + </thin_film_bsdf>
  + <layer name="thin_film_layer" type="BSDF">
  + <input name="top" type="BSDF" nodename="thin_film_bsdf" />
  + <input name="base" type="BSDF" nodename="metalness_mix" />
  + </layer>
  +
  <!-- Coat Layer -->
  <mix name="coat_attenuation" type="color3">
  <input name="fg" type="color3" interfacename="coat_color" />
  <input name="bg" type="color3" value="1.0, 1.0, 1.0" />
  <input name="mix" type="float" interfacename="coat" />
  </mix>
  - <multiply name="metalness_mix_attenuated" type="BSDF">
  - <input name="in1" type="BSDF" nodename="metalness_mix" />
  + <multiply name="thin_film_layer_attenuated" type="BSDF">
  + <input name="in1" type="BSDF" nodename="thin_film_layer" />
  <input name="in2" type="color3" nodename="coat_attenuation" />
  </multiply>
  <roughness_anisotropy name="coat_roughness_vector" type="vector2">
  </dielectric_bsdf>
  <layer name="coat_layer" type="BSDF">
  <input name="top" type="BSDF" nodename="coat_bsdf" />
  - <input name="base" type="BSDF" nodename="metalness_mix_attenuated" />
  + <input name="base" type="BSDF" nodename="thin_film_layer_attenuated" />
  </layer>
  <!-- Emission Layer -->
  