wayland: Fix gdk_window_wayland_resize_cairo_surface()

Like in other backends (except X) we can't resize cairo image surfaces
so let's sync the code here with what the other backends do.

This prevents the painting machinery above us to paint on the wrong
buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=724968

gdk/wayland/gdkwindow-wayland.c

@@ -1487,7 +1487,10 @@ gdk_window_wayland_resize_cairo_surface (GdkWindow       *window,
                                          gint             width,
                                          gint             height)
 {
-  return surface;
+  /* cairo image surfaces cannot be resized */
+  cairo_surface_destroy (surface);
+
+  return NULL;
 }
 
 static cairo_region_t *