plugins: fix "current" video memory flags.
plugins: fix "current" video memory flags.

If the surface proxy is updated into the GstVaapiVideoMemory, then
it is assumed it is the most current representation of the current
video frame. Likewise, make a few more arrangements to have the
"current " flags set more consistently.

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