Add OpenPBR scene variants

[theblackunknown]

Summary

This PR fixes #91 by adding a material_model variant set on various prims to select an OpenPBR surface shader instead of the existing surface shader.
We keep backward compatibility by setting material_model variants set to standard_surface to keep the legacy behavior and look.

For easy consumption a new "top layer" stage file standard_shader_ball_scene_openpbr.usda has been added.

Note that this PR keeps the current ways of specifying looks: when a MaterialX document was used, I have added another one for OpenPBR tagged with MaterialX v1.39, where UsdShade graphs were used I have simply added new nodes.
Ideally we should strive to create MaterialX document for mtlx nodegraphs so that MaterialX data versionning and upgrading cna be handled via usdMtlx.

Quick test of using standard_shader_ball_scene_openpbr.usda in Maya and trying some variants from example_material variant set

[crydalch]

Thanks for starting this up, great start! Some thoughts:

• We should be sure to name the variant "OpenPBR Surface" or "OpenPBRSurface" (i.e. we should include "surface" not just "OpenPBR")
• I'm not sure how I feel about having a separate layer that just sets the shading variant? It's easy enough to switch variants in usdview, and I don't feel like there's a lot of benefit to using variants, if we start adding separate layers that only set variants.
• I think having OpenPBR Surface be the default variant selection could makes sense; though it might be a bit premature, as mtlxstandardsurface is more widely supported to this point... Would be good to hear from others in the community on that point.
• We should also probably increase the version as well in the readme, to 1.1

Just some thoughts, but thanks again for starting this!

[theblackunknown]

Thanks for your review @crydalch !

• We should be sure to name the variant "OpenPBR Surface" or "OpenPBRSurface" (i.e. we should include "surface" not just "OpenPBR")

Good point, I have updated this (a8e705c)

• I'm not sure how I feel about having a separate layer that just sets the shading variant? It's easy enough to switch variants in usdview, and I don't feel like there's a lot of benefit to using variants, if we start adding separate layers that only set variants.

This was originally brough up as a convenience for all those applications which does not have good variant support.
I do not have a strong opinion on this, so I'll remove it for now. (c4de7e0)

• I think having OpenPBR Surface be the default variant selection could makes sense; though it might be a bit premature, as mtlxstandardsurface is more widely supported to this point... Would be good to hear from others in the community on that point.

I am personally fine to make the switch now, especially as we can have backward compatibility by a simple variant switch. (577a8c1)

What do you think ?

• We should also probably increase the version as well in the readme, to 1.1

Very good point, I have added it and other information about the update in the README (34b042b)