qt: probe vulkan before opengl on windows
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OpenGL support on Windows is notoriously bad. This is mostly relevant for Windows 7 where D3D11 is not available that OpenGL is picked.
Latest display drivers available for Windows 7 appear to be supporting Vulkan 1.0, which is what Qt reportedly uses.
Depends on !6793 (merged).
Request review @chub.
VideoLAN code repository instance