Support emissive strength in Standard Surface to glTF PBR (AcademySoftwareFoundation#2593)

closes: AcademySoftwareFoundation#2584

Instead of multiplying the emissive color (factor) and the emission (strength) in the translation graph, we now pass them through to the `gltf_shader`, which do the multiplication, since [KHR_materials_emissive_strength](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_emissive_strength) is implemented in `gltf_pbr`.

The main difference in the current implementation between standard surface and gltf_pbr for emission is that standard surface takes the coat into consideration ([as emission is below the coat layer](https://autodesk.github.io/standard-surface/#closures/emission)). The gltf spec also layers emission below the coat, however not implemented in `gltf_pbr` currently, more info in issue: AcademySoftwareFoundation#2592. Even if the coat interaction differs a bit, I think we can just pass the emission and emission color through as both the shading models otherwise have the same implementation.

Author: bowald

libraries/bxdf/translation/standard_surface_to_gltf_pbr.mtlx

@@ -35,6 +35,7 @@
     <output name="clearcoat_out" type="float" />
     <output name="clearcoat_roughness_out" type="float" />
     <output name="emissive_out" type="color3" />
+    <output name="emissive_strength_out" type="float" />
   </nodedef>
 
   <nodegraph name="NG_standard_surface_to_gltf_pbr" nodedef="ND_standard_surface_to_gltf_pbr">
@@ -188,10 +189,12 @@
     </dot>
 
     <!-- Emission -->
-    <multiply name="emissive" type="color3">
-      <input name="in1" type="color3" interfacename="emission_color" />
-      <input name="in2" type="float" interfacename="emission" />
-    </multiply>
+    <dot name="emissive" type="color3">
+      <input name="in" type="color3" interfacename="emission_color" />
+    </dot>
+    <dot name="emissive_strength" type="float">
+      <input name="in" type="float" interfacename="emission" />
+    </dot>
 
     <!-- Specular Index of Refraction -->
     <dot name="ior" type="float">
@@ -212,6 +215,7 @@
     <output name="clearcoat_out" type="float" nodename="clearcoat" />
     <output name="clearcoat_roughness_out" type="float" nodename="clearcoat_roughness" />
     <output name="emissive_out" type="color3" nodename="emissive" />
+    <output name="emissive_strength_out" type="float" nodename="emissive_strength" />
 
   </nodegraph>
 </materialx>