Add unit test framework for Vulkan

[doug-walker]

Now that OCIO 2.5 supports Vulkan, it would be great to test that in GitHub Actions. This would probably consist of several tasks, which could potentially be done by different people:

• Add to libutils a "vulkanapp" equivalent to the existing oglapp or metalapp. The sample code below may have the necessary pieces.
• Update tests/gpu/GPUUnitTest.cpp to add an option to execute the existing tests through that renderer (should be trivial).
• Update the GitHub GPU Action to do whatever magic is needed to make Vulkan run properly in the virtualized instance.

JF provided the following example, which may help. "This code sample apparently demonstrates headless rendering in Vulkan, which is supported natively:"
https://github.com/SaschaWillems/Vulkan/blob/master/examples/renderheadless/renderheadless.cpp

[pmady]

I'd like to work on this issue. Based on my analysis, I'll implement the Vulkan unit test framework in the following phases:

Phase 1: Create vulkanapp in libutils

• Add vulkanapp.h and vulkanapp.cpp to src/libutils/oglapphelpers/
• Implement headless Vulkan rendering similar to the existing oglapp structure
• Support the same interface as OglApp for easy integration

Phase 2: Update GPUUnitTest.cpp

• Add Vulkan as a renderer option alongside OpenGL and Metal
• Minimal changes needed since the test framework is already abstracted

Phase 3: Update GitHub Actions

• Configure the GPU workflow to support Vulkan testing
• Install Vulkan SDK and configure headless rendering in CI

I'll start with Phase 1 and create a PR for review. Let me know if this approach works or if you have any specific requirements!