Add RMStream object as function argument to public functions so that non-.rm

AVStreams can be used to call these public rmdec.c functions as well, as is
the case for RDT/RTSP streams. See mailinglist discussion in "[PATCH] rdt.c:
don't reuse the same AVStream in both RTSP and RM demuxer" thread.

Originally committed as revision 16366 to svn://svn.ffmpeg.org/ffmpeg/trunk