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XR: Re-enable SteamVR OpenGL backend for AMD gpus

Addresses T76082.

Since the DirectX backend does not work for AMD gpus
(wglDXRegisterObjectNV() fails to register the shared OpenGL-DirectX
render buffer, displaying a pink screen to the user), the original
solution was to use SteamVR's OpenGL backend, which, as tested
recently, seems to work without any issues on AMD hardware.

However, the SteamVR OpenGL backend (on Windows) was disabled in
fe492d922d6d since it resulted in crashes with NVIDIA gpus (and still
crashes, as tested recently), so SteamVR would always use the
AMD-incompatible DirectX backend (on Windows).

This patch restores use of the SteamVR OpenGL backend for non-NVIDIA
(AMD, etc.) gpus while maintaining the DirectX workaround for NVIDIA
gpus. In this way, issues are still resolved on the NVIDIA side but
AMD users can once again use the SteamVR runtime, which may be their
only viable option of using Blender in VR.

Reviewed By: Julian Eisel

Differential Revision: https://developer.blender.org/D12409