Remove number suffix from 'color' and 'opacity' primvars

This is in line with the previous texcoord primvar change.

docs/Structure_Mapping.md

@@ -135,10 +135,10 @@ Depending on material parameters and available accessors, guc creates following
 
 Name | Description
 ---  | ---
-colorN | Vertex color set N
+color[N] | Vertex color set N
 displayColor | Display color (constant or per-vertex)
 displayOpacity | Display opacity (constant or per-vertex)
-opacityN | Vertex opacity set N
+opacity[N] | Vertex opacity set N
 st[N] | Texture coordinate set N
 tangents | Three-component tangent vectors
 bitangentSigns | Bitangent handedness

src/libguc/src/naming.cpp

@@ -79,14 +79,22 @@ namespace guc
     // Furthermore, 'color' maps directly to the COLOR_ glTF attribute name and goes well
     // with the already existing 'displayColor' primvar. It's just not for the 'display'
     // purpose, but rather part of the actual data used for shading.
-    std::string colorSetBaseName = "color";
-    return colorSetBaseName + std::to_string(index);
+    std::string name = "color";
+    if (index == 0)
+    {
+      return name;
+    }
+    return name + std::to_string(index);
   }
 
   std::string makeOpacitySetName(int index)
   {
-    std::string opacitySetBaseName = "opacity";
-    return opacitySetBaseName + std::to_string(index);
+    std::string name = "opacity";
+    if (index == 0)
+    {
+      return name;
+    }
+    return name + std::to_string(index);
   }
 
   const static std::unordered_set<std::string> MTLX_TYPE_NAME_SET = {