Description of Change(s)

• Add PXR_ENABLE_EXPERIMENTAL_TESTS (off by default) to enable more graphics tests.
  • Building with it enabled, you now have access to way more tests.
  • Currently only used on macOS. I'll have Windows support ready a bit later.
• Cleanup use of PXR_ENABLE_GL_SUPPORT and PXR_BUILD_GPU_SUPPORT.
  • PXR_BUILD_GPU_SUPPORT is only true if PXR_ENABLE_GL_SUPPORT is true: cleanup redundant checks.
  • Add or correct various PXR_ENABLE_GL_SUPPORT checks.
• Remove X11 requirement.
  • Create a null GL context when X11 isn't available, so tests can still run (but without HgiGL support).
• Various changes to make HgiVulkan work on macOS. Using Lavapipe you can run the full test suite.
  • Enable GLSL support on macOS. This will also be needed by HgiWebGPU.
  • Add OpenGL 1.4 support to HgiInterop and use a VertexArray, as required by Apple OpenGL.
  • The vast majority of tests are passing. Most are failing because tiny differences compared to the baseline, minor issues with Lavapipe, or unreliable tests (mostly dependency on the order of unordered collection).
  • This PR focuses on getting the tests to run, not to pass.
• Fix testHdStCodeGen and testHdStQuadrangulation crashing due to undefined static destruction order.

Link to proposal (if applicable)

• N/A

Fixes Issue(s)

• N/A

Checklist

[ ] I have created this PR based on the dev branch

[X] I have followed the coding conventions

[X] I have added unit tests that exercise this functionality (Reference:
testing guidelines)

[X] I have verified that all unit tests pass with the proposed changes

[X] I have submitted a signed Contributor License Agreement (Reference:
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