add dts extractor #61

[Curid]

This is a rust port of the gortsplib dts extractor. I don't really understand how it works, but my test stream doesn't stutter anymore.

I know you're busy, I don't expect this to be reviewed anytime soon.

[scottlamb]

First, thank you!

I don't own any cameras that produce B frames, so...

• What camera are you using? If it's fairly cheap I might just buy one.
• Have you tried it with any pure-software RTSP source? I'll try the procedure I was looking at in https://github.com/scottlamb/retina/issues/61#issuecomment-1264538233 when I get a chance.

and...

I don't really understand how it works, but my test stream doesn't stutter anymore.

I don't really either and need to make another pass through to develop that understanding and compare this technique with gstreamer's h264timestamper [edit: code here]. Haven't looked closely enough yet to see if gstreamer and rtsp-simple-server (now called mediamtx apparently) are doing the same thing (and, if not, which is better). But I made one pass through and left some comments anyway.

[Curid]

The gstreamer implementation seems to add latency. I'm not sure if it's just some initial delay like with the MediaMTX implementation but otherwise that's a problem.

In order to determine the DTS of each frame, this element may need to hold back a few frames in case the codec data indicates that frame reordering is allowed for the given stream. That means this element may introduce additional latency for the DTS decision.

[Curid]

Found an explanation for how this algorithm works: https://github.com/bluenviron/mediamtx/issues/1002#issuecomment-2115930927

[Curid]

Would it be better to first add the dts extractor as separate module that people can use manually before integrating it into the Depacketizer?