rtsp-stream: Use seqnum-offset for rtpinfo
The RTP sequence number in the rtpinfo should be the first sequence number that the client can expect from the RTSP server. The current value is fetched 1) reading the last sample on the sink element, or 2) read the "stats" property on the payloader (in case it is present) and 3) the seqnum property from the payloader.
The seqnum property on the payloader is currently not correct if a buffer list was pushed. It should reflect the sequence number of the last pushed buffer, but for a list it will be the first buffer from the list. It should be the last buffer. If that is fixed, the rtpinfo will not be generated correctly anymore.
To prepare gst-rtsp-server for a seqnum property fix, it is changed to read the "seqnum-offset" from the "stats" property. If there are no "stats" property seqnum is still read.