Update UsdPreviewSurface Nodegraph

[klucknav]

This PR fixes the cutout issue described in gitIssue #2044. It also updates the nodegraph to reflect the recently updated PreviewSurface v2.6 which removes specular highlights for fully transparent materials.

[jstone-lucasfilm]

This looks good to me, @klucknav, and thanks for the updates!

[ld-kerley]

Do we need to retain the previous version as version=2.3 and add this as a version=2.6?

I think I recall @crydalch mentioning that he's seen people using UsdPreviewSurface for rendering glass by setting opacity=0 and relying on the specular response remaining.

I'm in favor of this change in the UsdPreviewSurface spec, as it aligns its behavior with other shading models, but I wonder if MaterialX consumers might get caught out by the change, and retaining the older version would allow people to ensure an older look would still be achievable?

[jstone-lucasfilm]

@ld-kerley Karen and I were just discussing this, but in my local testing, glass materials with zero opacity (i.e. maximum transmissive strength) render in the same fashion as before. That seems consistent with other industry shading models such as glTF PBR and Standard Surface, but if it's not the intent for UsdPreviewSurface, then additional updates may be needed.