Ok, the idea is to create an utilitiy to allow doing plan B SDP O/A on unified plan peerconnections.
The idea is similar to https://github.com/jitsi/sdp-interop and we will borrow several ideas from them.
We assume that bundling and rtcp-mux is used always.
#CASE A: unified calls plan B
In this case browser creates an SDP with one m-line per track. We are going to take that SDP, and create one unified local offer to be used in the PC setLocalDescription and one plan-B offer to send to the remote peer.