cursor-renderer/native: Fetch instead of pass wl_buffer when realizing

This is the next step in centralizing the cursor sprite hw sprite
realization paths.

https://gitlab.gnome.org/GNOME/mutter/issues/77

src/wayland/meta-wayland-buffer.h

@@ -68,6 +68,7 @@ G_DECLARE_FINAL_TYPE (MetaWaylandBuffer, meta_wayland_buffer,
                       META, WAYLAND_BUFFER, GObject);
 
 MetaWaylandBuffer *     meta_wayland_buffer_from_resource       (struct wl_resource    *resource);
+struct wl_resource *    meta_wayland_buffer_get_resource        (MetaWaylandBuffer     *buffer);
 gboolean                meta_wayland_buffer_attach              (MetaWaylandBuffer     *buffer,
                                                                  GError               **error);
 CoglTexture *           meta_wayland_buffer_get_texture         (MetaWaylandBuffer     *buffer);