message: Clarify opacity logic in Standard Surface
  This changelist clarifies the opacity logic in the Standard Surface nodegraph, replacing a <swizzle> node containing an unused "param" element with the equivalent channel-specific connection.
diff: <luminance name="opacity_luminance" type="color3">
  <input name="in" type="color3" interfacename="opacity" />
  </luminance>
  - <swizzle name="opacity_luminance_r" type="float">
  - <input name="in" type="color3" nodename="opacity_luminance" />
  - <param name="channels" type="string" value="r" />
  - </swizzle>
  <surface name="shader_constructor" type="surfaceshader">
  <input name="bsdf" type="BSDF" nodename="coat_layer" />
  <input name="edf" type="EDF" nodename="emission_edf" />
  - <input name="opacity" type="float" nodename="opacity_luminance_r" />
  + <input name="opacity" type="float" nodename="opacity_luminance" channels="r" />
  </surface>
  <!-- Output -->
  