Support Specular and Specular Color in Standard Surface to glTF PBR (AcademySoftwareFoundation#2623)

closes AcademySoftwareFoundation#2581 

## Summary
This PR passthrough the specular weight and the specular color from standard surface to gltf_pbr. 

This translation will not produce completely matching result, even if specular and specular_color control similar aspects in both the shading models, there are some fundamental differences. It will successfully translate the specular weight, so if you have a material with roughness 0, and specular 0, it will be a diffuse material (similar to roughness 1). But the difference will be in the specular_color, both for dielectric materials (non-metals) and conductors (metals).

Author: bowald

libraries/bxdf/translation/standard_surface_to_gltf_pbr.mtlx

@@ -5,6 +5,8 @@
     <input name="base" type="float" value="1" />
     <input name="base_color" type="color3" value="0.8, 0.8, 0.8" />
     <input name="metalness" type="float" value="0" />
+    <input name="specular" type="float" value="1" />
+    <input name="specular_color" type="color3" value="1, 1, 1" />
     <input name="specular_anisotropy" type="float" value="0.0" />
     <input name="specular_rotation" type="float" value="0.0" />
     <input name="specular_roughness" type="float" value="0.2" />
@@ -27,6 +29,8 @@
     <output name="base_color_out" type="color3" />
     <output name="metallic_out" type="float" />
     <output name="roughness_out" type="float" />
+    <output name="specular_out" type="float" />
+    <output name="specular_color_out" type="color3" />
     <output name="ior_out" type="float" />
     <output name="anisotropy_strength_out" type="float" />
     <output name="anisotropy_rotation_out" type="float" />
@@ -203,6 +207,14 @@
       <input name="in" type="float" interfacename="emission" />
     </dot>
 
+    <!-- Specular -->
+    <dot name="specular" type="float">
+      <input name="in" type="float" interfacename="specular" />
+    </dot>
+    <dot name="specular_color" type="color3">
+      <input name="in" type="color3" interfacename="specular_color" />
+    </dot>
+
     <!-- Specular Index of Refraction -->
     <dot name="ior" type="float">
       <input name="in" type="float" interfacename="specular_IOR" />
@@ -236,6 +248,8 @@
     <output name="base_color_out" type="color3" nodename="base_color" />
     <output name="metallic_out" type="float" nodename="metallic" />
     <output name="roughness_out" type="float" nodename="roughness" />
+    <output name="specular_out" type="float" nodename="specular" />
+    <output name="specular_color_out" type="color3" nodename="specular_color" />
     <output name="ior_out" type="float" nodename="ior" />
     <output name="anisotropy_strength_out" type="float" nodename="anisotropy_strength" />
     <output name="anisotropy_rotation_out" type="float" nodename="anisotropy_rotation" />