How can I generate Mp4Sample?

I would like to write an mp4 stream to ffmpeg's stdin (using pipes). How can I generate [Mp4Sample](https://docs.rs/mp4/0.14.0/mp4/struct.Mp4Sample.html#)-s to [write_sample](https://docs.rs/mp4/0.14.0/mp4/struct.Mp4Writer.html#method.write_sample) them into a stream?