Support Dispersion in Standard Surface to glTF PBR (#2636)

closes #2585 

This PR adds `dispersion` input to `gltf_pbr` and a translation from `transmission_dispersion` in standard surface to glTFs equivalent.

> [!NOTE] 
>Dispersion in this PR is only an input to **gltf_pbr**, it is not implemented in any render backends.
>This is similar how dispersion is treated in **OpenPBR** and **Standard surface**, as they are also missing implementations. For more info, check Issue: #2202 and PR: #2203.

The motivation to add this is to be able to bake dispersion inputs into gltf_pbr even if we can't render this in the MaterialX Viewer.

Author: bowald

libraries/bxdf/gltf_pbr.mtlx

@@ -29,6 +29,7 @@
     <input name="attenuation_color" uniform="true" type="color3" value="1, 1, 1" uimin="0, 0, 0" uimax="1, 1, 1" uiname="Attenuation Color" uifolder="Volume" />
     <input name="anisotropy_strength" type="float" value="0" uimin="0" uimax="1" uiname="Anisotropy Strength" uifolder="Anisotropy" />
     <input name="anisotropy_rotation" type="float" value="0" uimin="0" uimax="6.283185" uiname="Anisotropy Rotation" uifolder="Anisotropy" />
+    <input name="dispersion" type="float" value="0" uimin="0" uisoftmax="1.0" uiname="Dispersion" uifolder="Dispersion" />
     <output name="out" type="surfaceshader" />
   </nodedef>
 

libraries/bxdf/translation/standard_surface_to_gltf_pbr.mtlx

@@ -14,6 +14,7 @@
     <input name="transmission" type="float" value="0" />
     <input name="transmission_color" type="color3" value="1, 1, 1" />
     <input name="transmission_depth" type="float" value="0" />
+    <input name="transmission_dispersion" type="float" value="0" />
     <input name="sheen" type="float" value="0" />
     <input name="sheen_color" type="color3" value="1, 1, 1" />
     <input name="sheen_roughness" type="float" value="0.3" />
@@ -43,6 +44,7 @@
     <output name="sheen_roughness_out" type="float" />
     <output name="clearcoat_out" type="float" />
     <output name="clearcoat_roughness_out" type="float" />
+    <output name="dispersion_out" type="float" />
     <output name="emissive_out" type="color3" />
     <output name="emissive_strength_out" type="float" />
     <output name="normal_out" type="vector3" />
@@ -234,6 +236,19 @@
       <input name="in" type="float" interfacename="specular_IOR" />
     </dot>
 
+    <!-- Dispersion -->
+    <divide name="dispersion_from_abbe" type="float">
+      <input name="in1" type="float" value="20.0" />
+      <input name="in2" type="float" interfacename="transmission_dispersion" />
+    </divide>
+
+    <ifequal name="dispersion" type="float">
+      <input name="value1" type="float" interfacename="transmission_dispersion" />
+      <input name="value2" type="float" value="0.0" />
+      <input name="in1" type="float" value="0.0" />
+      <input name="in2" type="float" nodename="dispersion_from_abbe" />
+    </ifequal>
+
     <!-- Geometry -->
     <dot name="normal" type="vector3">
       <input name="in" type="vector3" interfacename="normal" />
@@ -275,6 +290,7 @@
     <output name="sheen_roughness_out" type="float" nodename="sqrt_sheen_roughness" />
     <output name="clearcoat_out" type="float" nodename="clearcoat" />
     <output name="clearcoat_roughness_out" type="float" nodename="clearcoat_roughness" />
+    <output name="dispersion_out" type="float" nodename="dispersion" />
     <output name="emissive_out" type="color3" nodename="emissive" />
     <output name="emissive_strength_out" type="float" nodename="emissive_strength" />
     <output name="normal_out" type="vector3" nodename="normal" />