Add a flags field to the RTPDynamicPayloadPacketHandlerProc (PKT_FLAG_*).
This can be used later by RDT to get the flags from the RTP packet and
use that for the RealMedia packet (such as whether this RTP packet
represents a keyframe or not). For discussion, see "[PATCH] Realmedia
/ RTSP (RDT)".
Originally committed as revision 11557 to svn://svn.ffmpeg.org/ffmpeg/trunk