ci-vfxall: rely on OpenUSD MaterialX resources

[nicolaspopravka]

Remove the global PXR_MTLX_STDLIB_SEARCH_PATHS workaround from ci-vfxall.

OpenUSD 26.03 and newer include a fix added by PR #2904 (PixarAnimationStudios/OpenUSD#2904). It updates usdMtlx’s CMakeLists.txt so the MaterialX standard library is installed with the usdMtlx plugin and can be found without the environment variable. I think it is ok for the issue to remain in the older CYs instead of bringing the workaround into newer CYs.

Testing confirmed:

• CY2023–2025 require the workaround.
• CY2026–2027 work without it.

Test results: https://github.com/nicolaspopravka/aswf-docker/actions/runs/30363851174

Follow-up to #452; revisits #451.

Assisted-by: OpenAI Codex / GPT-5

[jfpanisset]

OpenUSD 26.03 went into the images:

ci-2026-clang19.4
ci-2026-clang20.4

The images:

ci-2027-clang21.0
ci-2027-clang22.0

have 26.05, although for the "real" release of 2027 images I'm thinking of going to 26.08.

Do you know which version of ci-vfxall you were testing against where you were seeing MaterialX related failures? And are you OK with MaterialX in OpenUSD support only working with current 2026 / 2027 ci-vfxall images?

[nicolaspopravka]

Thanks. Yes, I’m ok with MtlX support without the environment workaround only in CY2026/CY2027 images, and with leaving the older CY images as-is.

I tested the aswf/ci-vfxall images for all CYs, 2023 to 2027. OpenChessSet renders with fallback/grey materials in each CY, so unfortunately I see a MaterialX rendering failure across the entire CY range.

The exact failures differ : CY2026 and CY2027 explicitly report the missing libraries/stdlib/genglsl/lib/mx_math.glsl include; earlier years render the grey-shaded image even if the render command returning success.

The specific issue reported in #454 is the CY2027 run using aswf/ci-vfxall:2027.

Moving the "real" CY2027 images to OpenUSD 26.08 sounds good. Happy to rerun the same benchmark scene against that image when it's available.