Attached is a patch and a c file that uses ffmpeg's rtsp support to create an
rtspsrc.  Currently only mpeg4 video works, but aac audio is not too far away.

Issues that Ronald has pointed out:

i) the gst_ffmpegrtspsrc_averror() function needs to be moved to gstffmpeg.c,
with a declaration in gstffmpeg.h, since it's shared code between rtspsrc/demux
ii) you add mpeg4 to the template caps,you shouldn'd do that. it implies a bug
in gstffmpegcodecmap.c, I'd rather see you fix that
iii) demux events need to be added so seeks work (both in rtspsrc and in demux)