RTP: when sending to RTP an odd port we need to specify a=rtcp

Otherwise the receiver would -in principle- try to receive RTP on the
next (plus one) even port, whereas the m= line port would be used for
RTCP. We could also pass a=rtcp for the normal even/RTP & odd/RTCP
ports setup, but that can only confuse receivers with no gain.

Note that receivers may not parse the a=rtcp parameter in any case. In
fact, VLC does not parse the a=rtcp field to this day. In any case,
sending RTP to an *even* port is the only safely interoperable option.