GstVaapiDecode is a descendant of GstVaapiMiniObject, so, thought we should
use its methods, even though it doesn't change functionality.
GstVaapiPixmap, GstVaapiTexture and GstVaapiWindow are descendant of
GstVaapiObject, hence its methods shall be used.
https://bugzilla.gnome.org/show_bug.cgi?id=772554
Make sure that libgstvaapi private headers remain internally used to
build libgstvaapi libraries only. All header dependencies were reviewed
and checks for IN_LIBGSTVAAPI definition were added accordingly.
Also rename GST_VAAPI_CORE definition to IN_LIBGSTVAAPI_CORE to keep
consistency.
Add API to transfer VA urfaces to native pixmaps. Also add an API to
render a native pixmap, for completeness. In general, rendering to
pixmap would only be useful to certain VA drivers and use cases on
X11 display servers. e.g. GLX_EXT_texture_from_pixmap (TFP) handled
in an upper layer.
