Adopt OpenPBR in StandardShaderBall scene's materials

[crydalch]

Once OpenPBR and MaterialX 1.39.X are all working with USD, we should consider updating the examples here to use that instead of standard_surface.

[meshula]

Probably makes sense to have 1.39 variant in order to make the scene available as soon as possible? Moving the stock build of USD to 1.39 will take some time, but USD should be buildable with 1.39 before it's the standard supported version.