RE: replaceTrack proposal

Stefan said: 

"Also, just checking, are the RTP guys OK with switching source (I had
the impression that there was a tight correlation between a camera and
an SSRC, but may remember wrongly)?"

[BA] I would say that an RTCRtpSender and the sending SSRC are closely related.  So if both cameras were sending at once, you'd have two RTCRtpSender objects, each with different SSRCs. 
However, I don't think there is a close relationship between an MST and an SSRC - the SSRC really only comes into play once the MST is encoded (e.g. that's why we have RTCRtpEncodingParameters.ssrc).