Change function prototype of the sdp_parse_a_line in DynamicProtocolHandler.

Change function prototype of the sdp_parse_a_line in DynamicProtocolHandler.
This function is called in rtsp.c for each a= line in the SDP of the Describe
response after m= RTSP stream descriptors. The function prototype used to
take an AVStream argument. For RDT, however, every RTSPStream represents
a set of streams of identical content, and can thus represent multiple
AVStreams. Therefore, it should not take an AVStream as argument. This
patch modifies it to accept a AVFormatContext (of the RTSP/SDP demuxer)
instead. See discussion in "[PATCH/RFC] change function prototype of
parse_sdp_a_line" thread on ML.

Originally committed as revision 16024 to svn://svn.ffmpeg.org/ffmpeg/trunk