isTransparentSurface() does not work for OpenPBR about materialx

[kwokcb]

Shouldn't we be tagging the inputs accordingly based on the spec otherwise this is very "brittle" ?

[JGamache-autodesk]

Definitely. I was not aware of the hint metadata until you pointed it to me. This is going to solve many issues.

[JGamache-autodesk]

I don't see any new code in isTransparentShaderNode() that uses those hints though. And none of the surfaces in bxdf use the new spec. I guess the feature did not make it in 1.39.1.

[kwokcb]

I guess not.

I'd suggest the following interface changes:

1. Add a Input.hasTransparencyHint() and / or Input.hasOpaqueHint() and setters for completeness.
2. Add a Node.getOpaqueHintList() on the node which can scan using methods in 1. to build a "opaque comparison" dictionary.
3. Call getOpaqueHintList() when testing

[JGamache-autodesk]

Looks very good. Adding a bit of caching at the NodeDef level would help to make sure we do not rescan the list of inputs every time looking for the ones with hints.

[kwokcb]

I did not add in caching to keep this the logic for generic "hint" accessors and the transparency checking logic localized in the util.