CI : Support rendering using Metal for Mac

[kwokcb]

Requirements

• Update Github actions workflow to allow for headless rendering using Metal on Mac
• The current entry points used for rendering are MaterialXView and MaterialXGraphEditor. Each can be incrementally added to CI.
• Note: MaterialXTest render unit tests have never been added to CI so is not part of the scope of this issues.
• There are a few tests which work on runners without GPUs (software) which run for Linux only.

Proposed changes

• Add test_render option to appropriate runners.
• Add setup to allow software rendering.

  - name: Initialize Headless Rendering (macOS)
      if: matrix.test_render == 'ON' && runner.os == 'macOS'
      run: |
        # Force software rendering for Metal backend 
        echo "MTL_HARDWARE_RENDERING=0" >> $GITHUB_ENV
        # Enable Metal debug layer for better debugging in CI
        echo "MTL_DEBUG_LAYER=1" >> $GITHUB_ENV
        # Force software rendering for OpenGL backend compatibility
        echo "LIBGL_ALWAYS_SOFTWARE=1" >> $GITHUB_ENV

MaterialXView

This appears to work for MaterialXView with these results:

MaterialXGraphEditor

Does not run with this error:

MaterialXGraphEditor[18931:75242] Metal API Validation Enabled
Glfw Error 65545: NSGL: Failed to find a suitable pixel format

Root cause analysys is required to determine the cuase. It appears to be ImGUI setup related?