Instead of passing along the system frame number, we pass along a reference to
the GstVideoCodecFrame.
In order for all internal cleanup to happen properly, this also requires
switching to the proper AVPacket creation/destruction methods (av_packet_alloc()
and av_packet_unref())
Fixes#2568
Co-authored-by: Edward Hervey <edward@centricular.com>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5186>