rtsp-stream: collect rtp info when blocking
We don't unblock the stream anymore before replying to the play request (883ddc72), so the sinks don't have a last-sample after potentially flush seeking. seek_trickmode waits for preroll however, which means the stream will block and wait for a first buffer. Subsequent calls to get_rtpinfo() can thus make use of the information.
See #115 (closed)