Fix video filename with ffmpeg writer

[SuuperW]

Issue fixed:

1. Set window size to 2x.
2. Enable A/V capture OSD
3. Record video with ffmpeg writer, give the name "test.mp4"
4. Frame advance to actually capture a frame
   Result: The video is written to "test_1.mp4". File "test.mp4" was created but has no video.

This PR fixes the issue by pulling out the logic for acquiring a IVideoProvider that was being used in MainForm.AvFrameAdvance to its own method, and using that method to obtain the initial video resolution.

Check if completed:

• I have run any relevant test suites
• I, the commit author, have read the licensing terms for contributors (last updated 2024-06-22) and am compliant

[nattthebear]

I could never follow the mainform shenanigans for frame advance event ordering and similar stuff very well.

This seems reasonable.

resolved