Add Simulcast support

[Sean-Der]

This allow senders to send multiple quality levels as one stream.

I also added an example. It just works against the public WHIP/WHEP server I run. I will adjust to w/e you think is best @paullouisageneau. I need to make everything argv driven, but currently you can

• run example
• ffmpeg -re -f lavfi -i testsrc=size=640x480:rate=30 -pix_fmt yuv420p -c:v libx264 -g 10 -preset ultrafast -tune zerolatency -f rtp -payload_type 96 'rtp://127.0.0.1:6000?pkt_size=1200
• Load https://b.siobud.com/seanTest

Then you will see a drop down where you can select different quality levels.

[Sean-Der]

@paullouisageneau Ok everything is green!

• What additional tests should I add? I will add some simple unit tests, but do you have anything in particular?
• Are you ok with this API?
• How would you like the example to flow
• Anything else?

[paullouisageneau]

Thank you for the addition. It looks good overall, I'll look into it over the weekend.

[Sean-Der]

Nice! If the C++ API looks ok will add the C equivalent

Hopefully I kept the new API/designs in spirit with current. Don’t want to just bolt something ugly on the side :(

[murillo128]

Not sure how RTX is handled in libdatachannel, but rtx packets should use the repaired-rtp-stream-id not the rtp-stream-id, is it possible to be handled with this API?

[paullouisageneau]

Wouldn't it be easier to implement support for rids inside Description::Media and use the same track for the different quality levels?

[paullouisageneau]

Not sure how RTX is handled in libdatachannel, but rtx packets should use the repaired-rtp-stream-id not the rtp-stream-id, is it possible to be handled with this API?

RTX are generated by RtcpNackResponder. I think this is another argument to use the same track for the different quality levels: otherwise, it may not be possible to properly leverage per-track media handlers for simulcast.

[Sean-Der]

Hey @paullouisageneau can I get another review please, thank you!

[paullouisageneau]

Thank you for the changes, this looks better to me, the implementation is way simpler!

[Sean-Der]

Hi @paullouisageneau responded to all the reviews. Mind taking a look again, thank you!

[Sean-Der]

@paullouisageneau done!

Pulled out the example also. PR is much smaller now :)

[Sean-Der]

@paullouisageneau sorry for all the mistakes :/

Pushed mind reviewing again?

[paullouisageneau]

It's looking good, thank you!

[pkviet]

@paullouisageneau could you create a tag for obs ? Unless you plan an imminent new release.
Thanks.
Btw we've bypassed the issue we were having with cross compiled DLLs by switching to a full msvc build.

[paullouisageneau]

@paullouisageneau could you create a tag for obs ? Unless you plan an imminent new release.
Thanks.

@pkviet I'll create a tag when a solution to #891 (comment) is merged, so this can be used with OBS.

Btw we've bypassed the issue we were having with cross compiled DLLs by switching to a full msvc build.

Great news!

[pkviet]

@paullouisageneau hi, pinging again for a new tag for obs !

[paullouisageneau]

@pkviet Sorry for the delay, it's now tagged as v0.19.0-alpha.4.