Add a new generic helper function gst_vaapi_window_new() to create
a window without having the caller to check for the display type
himself. i.e. internally, there is now a GstVaapiDisplayClass hook
to create windows, and the actual backend implementation fills it in.
Add new generic helper functions gst_vaapi_texture_new_wrapped()
This is a simplification in view to supporting EGL.
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.
