Function from_reader_with_timestamp_source 

pub fn from_reader_with_timestamp_source(
    mp4_reader: Mp4Reader<Box<dyn SeekRead + Send>>,
    do_decode_h264: bool,
    timestamp_source: TimestampSource,
    srt_file_path: Option<PathBuf>,
    show_progress: bool,
    preparser: Option<Box<dyn H264Preparser>>,
) -> Result<H264Source<Mp4Source>>

Construct a frame source from an already-opened MP4 reader.