Display progress of ffmpeg calls

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When yt-dlp calls ffmpeg, it uses ffmpeg.exe rather than their API. It then redirects the output from ffmpeg to the nul device. So the user has no idea how much longer ffmpeg is going to need. They might think it or yt-dlp hung. During some of these calls, if the user interrupts the process with Ctrl-C and then restarts it, yt-dlp thinks the call to ffmpeg completed successfully and proceeds to the next video.

I think the API should be used. That would then allow yt-dlp to display a progress via ffmpeg’s callback routines.

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