h264: add vaapi_fill_picture() helper.
h264: add vaapi_fill_picture() helper.

Add vaapi_fill_picture() helper function to convert GstVaapiPictureH264
to VAPictureH264 structure. This is preparatory work to get rid of the
local VAPictureH264 member in GstVaapiPictureH264.

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