I am recording from a Raspberry Pi Camera to an .h264 stream. I have each frame as well as its presentation timestamp. How can I convert this to an .mp4 file? Specifically I need to be able to provide the timestamp or duration of each frame, as the framerate is not necessarily constant. I know ffmpeg can convert .h264 to .mp4 with a constant framerate, but I don't see how to do it with a variable one.