EMSUSD-2188 make materialX bindings accessible in Maya

[pierrebai-adsk]

• Add the path leading to MaterialX to the PYTHONPATH in the MayaUSD Maya plugin module.
• Add a unit test that imports the MaterialX bindings.
• Make the unit test dependent on the version of USD
• Fix problem with GLSL in unit test in Maya 2025 with an extra import.

[JGamache-autodesk]

I do suspect you will have to make this version dependent on USD as these Python bindings are a recent addition to the USD we package with MayaUSD.

[pierrebai-adsk]

@JGamache-autodesk I'm getting errors on older version of MayaUSD. Are they expected? Should the test only be run for Maya 2026 and higher? For example:

    import MaterialX.PyMaterialXGenGlsl as mxgl
11:03:30      ImportError: generic_type: type "GlslShaderGenerator" referenced unknown base type "MaterialX_MayaUSD_v1_38_8::HwShaderGenerator"

On Maya 2022:

import MaterialX as mx
10:59:19      ModuleNotFoundError: No module named 'MaterialX'

[JGamache-autodesk]

I would expect 2025 and 2026 to work. Before that we did not even compile the MaterialX Python bindings.
The error in 2025 is annoying. I would have expected at least Linux to work. Can you try importing PyMaterialXGenShader before PyMaterialXGenGlsl to see if it helps resolving the base class of GlslShaderGenerator?

[pierrebai-adsk]

I tested on Windows, Maya 2025.3: adding that import fixes the problem.

[JGamache-autodesk]

Good!
Maya 2025 has MaterialX 1.38.8, while Maya 2026 has MaterialX 1.38.10-OpenPBR. Main difference is this very important PR AcademySoftwareFoundation/MaterialX#1595 that correctly pre loads dependencies in 1.38.10. Without it you need to manually load dependencies.