Use chunk-size in function calling mdpr_read_codecdata() rather than in the

Use chunk-size in function calling mdpr_read_codecdata() rather than in the
function itself. This way, old-style (.ra4) RTSP/RDT OpaqueData chunks can
be parsed using this function as well (they don't have the size bits). See
discussion on ML in "[PATCH] rmdec.c - small read_mdpr_codecdata() API
modification".

Originally committed as revision 15163 to svn://svn.ffmpeg.org/ffmpeg/trunk