Consider subsurface color in dielectric albedo (#2105)

I am experimenting with OpenPBR to UsdPreviewSurface export code in Maya and tried chaining the two translator graphs already provided in MaterialX. Results are quite good, except when the subsurface color is extremely different from the base color.

This can be fixed by computing a dielectric base albedo that mixes the scaled base color and the subsurface color.

libraries/bxdf/translation/standard_surface_to_usd.mtlx

@@ -12,6 +12,8 @@
     <input name="coat" type="float" value="0" />
     <input name="coat_color" type="color3" value="1, 1, 1" />
     <input name="coat_roughness" type="float" value="0.1" />
+    <input name="subsurface" type="float" value="0" />
+    <input name="subsurface_color" type="color3" value="1, 1, 1" />
     <input name="emission" type="float" value="0" />
     <input name="emission_color" type="color3" value="1, 1, 1" />
     <input name="opacity" type="color3" value="1, 1, 1" />
@@ -47,13 +49,18 @@
       <input name="in1" type="color3" interfacename="base_color" />
       <input name="in2" type="float" interfacename="base" />
     </multiply>
+    <mix name="albedoOpaqueDielectric" type="color3">
+      <input name="fg" type="color3" interfacename="subsurface_color" />
+      <input name="bg" type="color3" nodename="scaledBaseColor" />
+      <input name="mix" type="float" interfacename="subsurface" />
+    </mix>
     <mix name="coatAttenuation" 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="diffuseColor" type="color3">
-      <input name="in1" type="color3" nodename="scaledBaseColor" />
+      <input name="in1" type="color3" nodename="albedoOpaqueDielectric" />
       <input name="in2" type="color3" nodename="coatAttenuation" />
     </multiply>
     <dot name="roughness" type="float">