libs: decoder: h264: Add ref flags for splited field.
libs: decoder: h264: Add ref flags for splited field.

When split one frame into fields, the second field should also
copy the reference flags.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/333>

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi