Backport OpenPBR Surface to MaterialX 1.38.10

[JGamache-autodesk]

Backports only the minimal code necessary to render native OpenPBR surface in MaterialX in a way that has no visual impact on the existing shaders and pbr nodes present in 1.38.

Status:

• GLSL -> Fully functional
• MSL -> Fully functional. Required only backporting the inversesqrt fix on top of the GLSL code.
• OSL -> As functional as the 1.39 branch. Getting it fully functional requires OSL v1.14 features not yet released like Zeltner fuzz and compensating OrenNayar. I was not able to find any mention of a 82 deg color update on the OSL Schlick node though.
• MDL -> Compiles and validates, but no updated visuals until an updated MDL library is available.