Managing reference picture type by using two variables (ref and long_term) seems to be redundant and that can be represented by using a single enum value. This is to sync this implementation with gstreamer-vaapi so that make comparison between this and gstreamer-vaapi easier and also in order to minimize the change required for subclass to be able to support interlaced. Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1534>