Validate and Update for MaterialX v1.38.10-OpenPBR

[matthewlow-dwa]

As official support of OpenPBR via MaterialX 1.39 within OpenUSD is still a ways off (keeping this asset in beta), the OpenUSD and MaterialX teams have agreed to explore an interim compromise, by backpatching OpenPBR functionality to MaterialX 1.38 and are using the v1.38.10-OpenPBR tags to track possible OpenUSD adoption and industry integration:

• https://academysoftwarefdn.slack.com/archives/C0230LWBE2X/p1730233131919539
• https://github.com/AcademySoftwareFoundation/MaterialX/tree/v1.38.10-OpenPBR-rc1

Once these versions are qualified by the MaterialX and OpenUSD teams, this OpenPBR Shader Playground asset should be validated against those releases and updated as necessary for a possible v1.0 release candidate for DPEL.