v4l2codec: Pass system frame number as timestamp and use it to retrieve back frames...

v4l2codec: Pass system frame number as timestamp and use it to retrieve back frames reliably

System frame numbers are supposed to be unique and correct drivers are
passing through timestamps without modification from the output/sink to the
capture/src side.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/778>