This fixes a misbehaving proton game that sets MF_XVP_PLAYBACK_MODE.
MF_XVP_PLAYBACK_MODE isn't mentioned in microsoft docs, but it's explained and used in the MIT-licensed [Windows-classic-samples](https://github.com/microsoft/Windows-classic-samples/blob/2b94df5730177ec27e...)
Not sure if mfplat is the right place to add the tests for this since the code is in winegstreamer, but it's very similar to the existing mfplat tests and the test does not interface with winegstreamer directly.
It should be noted that on windows, ProcessOutput errors out if we provide a pSample with MF_XVP_PLAYBACK_MODE unset, wine simply ignores it.
-- v3: winegstreamer: Allow caller to allocate samples in MF_XVP_PLAYBACK_MODE. mfplat/tests: Add test for MF_XVP_PLAYBACK_MODE.