Add dynamic payload handlers to rdt.c. These follow the same API as the ones

Add dynamic payload handlers to rdt.c. These follow the same API as the ones
in rtpdec.c, so that they can be shared and used in the same way in rtsp.c.
The handlers, since they are specific for RDT, are registered in rdt.c and
a new registration function is thus called from allformats.c.

The dynamic payload handler also implements RDT-specific SDP-line parsing for
OpaqueData and StartTime, which are specific for RDT and needed for proper
playback. OpaqueData contains one or a list ("MLTI") of "MDPR" chunks that
can be parsed by the rmdec.c function ff_rm_read_mdpr_codecdata(). To use
this function, we create a new rdt_demuxer, which has the same private data
as the rm_demuxer. The resulting AVFormatContext created with _open_stream()
can thus be used to call functions in the RM demuxer.

See discussion in "Realmedia patch" thread on ML.

Originally committed as revision 15234 to svn://svn.ffmpeg.org/ffmpeg/trunk