Consolidate glTF PBR graph and remove GLSL override (#2850)

This changelist consolidates the dielectric iridescence path in the glTF PBR shading graph, and removes the hand-authored GLSL graph override that is no longer needed.  The following specific changes are included:

- Restructure the dielectric iridescence path from `mix(layer(A, C), layer(B, C), w)` to `layer(mix(A, B, w), C)`, eliminating a duplicated layer node that was a workaround for earlier codegen limitations.
- Remove the hand-authored GLSL graph override for `gltf_pbr`, as the `PremultipliedBsdfAddRefactor` pass now reproduces these optimizations automatically from the consolidated global graph.

Author: jstone-lucasfilm

libraries/bxdf/genglsl/gltf_pbr.mtlx

@@ -167,13 +167,7 @@
       <input name="mix" type="float" interfacename="transmission" />
     </mix>
 
-    <layer name="dielectric_bsdf" type="BSDF">
-      <input name="top" type="BSDF" nodename="reflection_bsdf" />
-      <input name="base" type="BSDF" nodename="transmission_mix" />
-    </layer>
-
-    <!-- Thin-film + Dielectric
-         Note: Due to limitations in codegen, the base layer BSDF is duplicated (#1035). -->
+    <!-- Thin-film + Dielectric -->
 
     <generalized_schlick_bsdf name="tf_reflection_bsdf" type="BSDF">
       <input name="color0" type="color3" nodename="dielectric_f0" />
@@ -186,18 +180,17 @@
       <input name="thinfilm_ior" type="float" interfacename="iridescence_ior" />
     </generalized_schlick_bsdf>
 
+    <mix name="mix_iridescent_dielectric_reflection" type="BSDF">
+      <input name="bg" type="BSDF" nodename="reflection_bsdf" />
+      <input name="fg" type="BSDF" nodename="tf_reflection_bsdf" />
+      <input name="mix" type="float" interfacename="iridescence" />
+    </mix>
 
-    <layer name="tf_dielectric_bsdf" type="BSDF">
-      <input name="top" type="BSDF" nodename="tf_reflection_bsdf" />
+    <layer name="iridescent_dielectric_bsdf" type="BSDF">
+      <input name="top" type="BSDF" nodename="mix_iridescent_dielectric_reflection" />
       <input name="base" type="BSDF" nodename="transmission_mix" />
     </layer>
 
-    <mix name="mix_iridescent_dielectric_bsdf" type="BSDF">
-      <input name="bg" type="BSDF" nodename="dielectric_bsdf" />
-      <input name="fg" type="BSDF" nodename="tf_dielectric_bsdf" />
-      <input name="mix" type="float" interfacename="iridescence" />
-    </mix>
-
     <!-- Metal -->
 
     <generalized_schlick_bsdf name="metal_bsdf" type="BSDF">
@@ -208,8 +201,7 @@
       <input name="tangent" type="vector3" nodename="selected_tangent" />
     </generalized_schlick_bsdf>
 
-    <!-- Thin-film + Metal
-         Note: Due to limitations in codegen, the base layer BSDF is duplicated (#1035). -->
+    <!-- Thin-film + Metal -->
 
     <generalized_schlick_bsdf name="tf_metal_bsdf" type="BSDF">
       <input name="color0" type="color3" interfacename="base_color" />
@@ -230,7 +222,7 @@
     <!-- Dielectric/metal mix -->
 
     <mix name="base_mix" type="BSDF">
-      <input name="bg" type="BSDF" nodename="mix_iridescent_dielectric_bsdf" />
+      <input name="bg" type="BSDF" nodename="iridescent_dielectric_bsdf" />
       <input name="fg" type="BSDF" nodename="mix_iridescent_metal_bsdf" />
       <input name="mix" type="float" interfacename="metallic" />
     </mix>