Support fragmented mp4s

[jerzywilczek]

HLS uses fragmented mp4s (fMP4). We need to make sure we support these reasonably well.

While doing this, I encountered a problem in the library we use for mp4 parsing and demuxing, which I fixed: alfg/mp4-rust#136

I also found two problems with membrane's mp4 implementation, for which I filed bug reports:

• [MP4] MP4 plugin has a test case with audio and video but the HLS playlist only lists video. membraneframework/membrane_core#759
• MP4 plugin generates incorrect sequence numbers for moof boxes membraneframework/membrane_core#765

The problem with this task is that it's hard to make sure fragmented mp4s are well supported. At this point, we should support mp4s produced by membrane, but I stumbled across some other fmp4s which do not work and I wasn't able to get them to work.

I think this task is more or less finished, and we should try fixing the mp4 libraries when we find a file that somebody wants to use which doesn't work.

More results of the research I did are in the description of this task: #406