bridge: Also allow RTCP muxing to be used if we're the initiator

The other side is going to assume that RTCP muxing is used and
does not wait for RTCP on a different port... while we as the
initiator send RTCP on a different port and nothing happens.



This fixes owr<->owr interop, and probably more. Also works with firefox<->owr with both sides being the initiator.