Switch from Standard Surface to OpenPBR as the exported MaterialX surface,
since this is the new standard more renderers are adopting and it more closely
matches the Principled BSDF implementation.
Anisotropy support is improved though still not quite the same, as formulas
are different. Nodes are generated to apply anisotropic rotation to the
tangent vector, as there is no corresponding parameter in OpenPBR.
Fixes#138164
Authored by Apple: Lee Kerley
Pull Request: https://projects.blender.org/blender/blender/pulls/138165
af0cfcb01c