MPVOpenGLLayer called fatalError() when CGLChoosePixelFormat failed for
both the 10-bit and 8-bit (kCGLPFAAccelerated) formats, killing the app at
launch on machines without a usable GPU such as virtual machines.
Add a non-accelerated software pixel format as a last resort and make the
whole OpenGL init path failable: createPixelFormat/createContext return nil
(with an error log) instead of aborting, MPVOpenGLLayer.init becomes init?,
and MPVOGLView holds an optional layer, throwing openGLSetupFailed from
setup(with:) when OpenGL is unavailable. That error is already handled by
MPVBackend.setupMPVAsync, so the app now launches cleanly and only logs that
video playback is unavailable.