[PATCH v3 4/5] winegstreamer: Implement IMFMediaSource::Start.

[Nikolay Sivov]

This one gives me two warnings:

../../wine-git/dlls/winegstreamer/media_source.c: In function
‘start_pipeline’:
../../wine-git/dlls/winegstreamer/media_source.c:309:24: warning: unused
variable ‘preroll’ [-Wunused-variable]
  309 |             GstSample *preroll;
      |                        ^~~~~~~
../../wine-git/dlls/winegstreamer/mfplat.c: In function
‘caps_from_mf_media_type’:
../../wine-git/dlls/winegstreamer/mfplat.c:620:16: warning: unused
variable ‘unused’ [-Wunused-variable]
  620 |         UINT32 unused;
      |                ^~~~~~