Duplicating a picture what was already a dup was leading to a crash. Rename the custom picture flags as HOLDS_BUFFER to make its meaning clear. Then save then ref and store the picture as userdata, so it can be obtained when duplicating. Finally, mark the doplicated as HOLDS_BUFFER to avoid thinking it holds a request. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1681>