Add "AVFormatContext *ctx" (that being the RTSP demuxer's) as first argument

Add "AVFormatContext *ctx" (that being the RTSP demuxer's) as first argument
to the parse_packet() function pointer in RTPDynamicProtocolHandlers. This
allows these functions to peek back and retrieve values from the demuxer's
context (or RTSPState). The ASF/RTP payload parser will use this to be able
to parse SDP values (which occur even before the payload ID is given), store
them in the RTSPState and then retrieve them while parsing payload data. See
"[PATCH] RTSP-MS 13/15: add RTSP demuxer AVFormatContext to parse_packet()
function pointer (was: transport context)" mailinglist thread.

Originally committed as revision 17015 to svn://svn.ffmpeg.org/ffmpeg/trunk