plugins: further fixes to the new "current" storage tracker.
plugins: further fixes to the new "current" storage tracker.

The ensure_surface() and ensure_image() functions shall only relate
to the underlying backing store. The actual current flags are to be
updated only through ensure_{surface,image}_is_current() or very other
particular cases in GstMemory hooks.

CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi