not use RTPDemuxContext, but rather take a pointer to the payload context
directly. This allows using payload handlers regardless over the transport
over which they were sent, and prepares for the introduction of a future
RDTDemuxContext. See discussion in "RDT/Realmedia patches #2" thread on ML.

Originally committed as revision 15541 to svn://svn.ffmpeg.org/ffmpeg/trunk