MaterialX 1.39.2 support

[boberfly]

Describe the issue When building OpenUSD 25.02a with MaterialX 1.39.x, I've found that due to the API changes it cannot build.

This is just slightly ahead of 1.39.2 release but I think it's due to the 1.38 to 1.39 changes that OpenUSD/usdMtlx needed to address.

Build log maya-usd.log

Specs:

• OS & version - Windows 10
• Compiler & version - Visual Studio 17 2022 v143
• Maya version - Maya 2025.3
• Maya USD commit - release/v0.31.0 (also testing dev branch now 804686e)
• Pixar USD commit - release (v25.02a)
• MaterialX 1.39.x (commit 95fc7c5)

Additional context Looks like I might be a bit ahead on things. I can take a look and see if I can add 1.39.2 support myself and make a PR.

[santosd]

Thank you for working on a PR for this one. Let us know if you have any issues or questions.

[boberfly]

Just to note https://github.com/Autodesk/maya-usd/pull/4160 addresses this, thank you.