rtsp-stream: collect rtp info when blocking

rtsp-stream: collect rtp info when blocking

We don't unblock the stream anymore before replying to the
play request (883ddc72bb5bc57c95a9e167814d1ac53fe1b443),
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 https://gitlab.freedesktop.org/gstreamer/gst-rtsp-server/-/issues/115

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-rtsp-server/-/merge_requests/160>