I want to get a very accurate wall clock time that the first frame in the
MP4 file corresponds to. This needs to be more accurate than just reading
the system time when I start the pipeline. I can sync the camera to an NTP
clock. The camera is supposed to send an NTP-to-RTP timestamp mapping in its
RTCP Sender Report. From this, gstreamer should know the NTP wall clock time
of the first frame. But how do I extract this information from gstreamer?