Function run_ffmpeg_with_input 

pub fn run_ffmpeg_with_input(
    args: &[String],
    output: &str,
    echo_stderr: bool,
    on_frame: &mut dyn FnMut(u32),
    input: Option<Box<dyn Read + Send>>,
) -> Result<String, String>

Like run_ffmpeg_with_progress, but feeds input to ffmpeg’s stdin (for an -i pipe:0 input, e.g. a PNG stream). The copy runs on its own thread, so a slow producer and ffmpeg can’t block each other. If reading input fails, the render fails too and the half-written output is removed.