Integrate OpenPBR update from MaterialX project (#265)

* Integrate OpenPBR updates from MaterialX project

This changelist integrates two post-1.1 updates to OpenPBR Surface from the MaterialX project:

- Optimizations to OpenPBR graph (AcademySoftwareFoundation/MaterialX#2459)
- Add code generation hints support (AcademySoftwareFoundation/MaterialX#1954)

The more substantial update is the graph optimization, and I've copied the performance measurements from the original change for reference:

Performance tests were conducted on an NVIDIA RTX A6000 at 4K resolution, and the following timing improvements were seen:

OpenPBR Carpaint: 16ms -> 7ms
OpenPBR Glass: 27ms -> 11ms
OpenPBR Pearl: 16ms -> 12ms
OpenPBR Aluminum: 14ms -> 5ms

* Omit hardware shading optimizations

Author: jstone-lucasfilm

reference/open_pbr_surface.mtlx

@@ -23,7 +23,7 @@
            doc="Index of refraction of the dielectric base." />
     <input name="specular_roughness_anisotropy" type="float" value="0.0" uimin="0.0" uimax="1.0" uiname="Specular Anisotropy" uifolder="Specular" uiadvanced="true"
            doc="The directional bias of the roughness of the metal/dielectric base, resulting in increasingly stretched highlights along the tangent direction." />
-    <input name="transmission_weight" type="float" value="0.0" uimin="0.0" uimax="1.0" uiname="Transmission Weight" uifolder="Transmission" uiadvanced="true"
+    <input name="transmission_weight" type="float" value="0.0" uimin="0.0" uimax="1.0" uiname="Transmission Weight" uifolder="Transmission" uiadvanced="true" hint="transparency"
            doc="Mixture weight between the transparent and opaque dielectric base. The greater the value the more transparent the material." />
     <input name="transmission_color" type="color3" value="1, 1, 1" uimin="0,0,0" uimax="1,1,1" uiname="Transmission Color" uifolder="Transmission" uiadvanced="true"
            doc="Controls color of the transparent base due to Beer's law volumetric absorption under the surface (reverts to a non-physical tint when transmission_depth is zero)." />
@@ -75,7 +75,7 @@
            doc="The amount of emitted light, as a luminance in nits." />
     <input name="emission_color" type="color3" value="1, 1, 1" uimin="0,0,0" uimax="1,1,1" uiname="Emission Color" uifolder="Emission"
            doc="The color of the emitted light." />
-    <input name="geometry_opacity" type="float" value="1" uimin="0" uimax="1" uiname="Opacity" uifolder="Geometry"
+    <input name="geometry_opacity" type="float" value="1" uimin="0" uimax="1" uiname="Opacity" uifolder="Geometry" hint="opacity"
            doc="The opacity of the entire material." />
     <input name="geometry_thin_walled" type="boolean" value="false" uiname="Thin Walled" uifolder="Geometry" uiadvanced="true"
            doc="If true the surface is double-sided and represents an infinitesimally thin shell. Suitable for extremely geometrically thin objects such as leaves or paper." />