Move video buffer creation routines to plugin elements. That exclusively uses *_typed_new*() variants.
Introduce new typed constructors internal to gstreamer-vaapi plugin elements. This avoids duplication of code, and makes it possible to further implement generic video buffer creation routines that automatically map to base or GLX variants.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>