Function run_ffmpeg_with_progress 

pub fn run_ffmpeg_with_progress(
    args: &[String],
    output: &str,
    echo_stderr: bool,
    on_frame: &mut dyn FnMut(u32),
) -> Result<String, String>

Run ffmpeg, calling on_frame with the number of frames written so far (from ffmpeg’s -progress output, about twice a second, and once at the end).

Returns ffmpeg’s stderr. With echo_stderr, ffmpeg’s log is also printed as it arrives. On failure a half-written output file is removed.