Folding More Standards Into OpenPBR

I think it would be great if other MaterialX-compatible texturing and rendering programs would fold their own texturing and shading standards into OpenPBR. To name several examples, ones from Pixar (RenderMan), Chaos (V-Ray), Maxon (ZBrush and Redshift), and Foundry (Mari).

This is where I learned it from.
[https://learn.foundry.com/mari/Content/reference_guide/shader_parameters/aistandard_surface.html](url)

Start by reviewing the OpenPBR specification and the linked Mari Standard Surface reference, then compare the cited RenderMan, V-Ray, ZBrush, Redshift, and Mari standards for concrete compatibility needs. Done would require an agreed scope and specification changes, but this issue does not define either.

Written by the indexing model from the issue text.