[TODO] Support OpenPBR

[syoyo]

https://github.com/AcademySoftwareFoundation/OpenPBR

Related to MaterialX support #86

We plan to write a reference implementation of OpenPBR shader on top of pbrlab https://github.com/lighttransport/pbrlab (by extending it to load USD model using TinyUSDZ)

[github-actions]

This issue is stale because it has been open for 30 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 14 days since being marked as stale.

[syoyo]

Merged into #86