This patch refactors RDT packet header parsing so that it can be used in
rtsp.c to detect the ID of the packet source also in case of TCP streams.
This allows proper playback of RDT streams with multiple stream types, e.g.
audio + video. Accepted by LucaB in "RDT/Realmedia patches #2" thread on ML.
Originally committed as revision 15496 to svn://svn.ffmpeg.org/ffmpeg/trunk