libavformat: Add a muxer wrapping mpegts encoding into RTP

libavformat: Add a muxer wrapping mpegts encoding into RTP

Since this structurally is quite different from normal RTP
(multiple streams are muxed into one single mpegts stream,
which is packetized into one single RTP session), it is kept
as a separate muxer.

Since this structurally also behaves differently than normal
RTP, all of the other muxers that do chained RTP muxing
(rtsp, sap, mp4) would need to be updated similarly to handle
this - in particular, creating one single rtp_mpegts muxer
for the whole presentation instead of one rtp muxer per stream.

Signed-off-by: Martin Storsjö <martin@martin.st>