plugins: fix creation of video buffer from another source buffer.
plugins: fix creation of video buffer from another source buffer.

gst_vaapi_video_buffer_new_from_buffer() needs to reference the source
buffer video meta since it would be unreference'd from the get_buffer()
helper function. For other cases, we still use (steal) the newly created
video meta.

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