subsurface: Add gdk_subsurface_get_sibling

This was just a missing getter. The backend implementation will
use this information to determine whether stacking changes.

gdk/gdksubsurface.c

@@ -286,6 +286,28 @@ gdk_subsurface_is_above_parent (GdkSubsurface *subsurface)
   return subsurface->above_parent;
 }
 
+/*< private>
+ * gdk_subsurface_get_sibling:
+ * @subsurface: the `GdkSubsurface`
+ * @above: whether to get the subsurface above
+ *
+ * Returns the subsurface above (or below) @subsurface in
+ * the stacking order.
+ *
+ * Returns: the sibling, or `NULL` if there is none.
+ */
+GdkSubsurface *
+gdk_subsurface_get_sibling (GdkSubsurface *subsurface,
+                            gboolean       above)
+{
+  g_return_val_if_fail (GDK_IS_SUBSURFACE (subsurface), NULL);
+
+  if (above)
+    return subsurface->sibling_above;
+  else
+    return subsurface->sibling_below;
+}
+
 /*< private >
  * gdk_subsurface_get_transform:
  * @subsurface: a `GdkSubsurface`

gdk/gdksubsurfaceprivate.h

@@ -101,6 +101,8 @@ void            gdk_subsurface_get_source_rect     (GdkSubsurface         *subsu
 void            gdk_subsurface_get_texture_rect    (GdkSubsurface         *subsurface,
                                                     graphene_rect_t       *rect);
 gboolean        gdk_subsurface_is_above_parent     (GdkSubsurface         *subsurface);
+GdkSubsurface * gdk_subsurface_get_sibling         (GdkSubsurface         *subsurface,
+                                                    gboolean               above);
 GdkTextureTransform
                 gdk_subsurface_get_transform       (GdkSubsurface         *subsurface);
 gboolean        gdk_subsurface_get_background_rect (GdkSubsurface         *subsurface,
@@ -108,6 +110,5 @@ gboolean        gdk_subsurface_get_background_rect (GdkSubsurface         *subsu
 void            gdk_subsurface_get_bounds          (GdkSubsurface         *subsurface,
                                                     graphene_rect_t       *bounds);
 
-
 G_END_DECLS