[gtk+] gdkwindow: Make the precondition of gdk_window_add_damage clearer

[Jasper St. Pierre <jstpierre src gnome org>]

commit 8eb092ea14b2ba4929784e8316a30e61b7fae4ab
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Mon Jun 23 12:29:55 2014 -0400

    gdkwindow: Make the precondition of gdk_window_add_damage clearer

 gdk/gdkwindow.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 67049b8..9cec667 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -3223,11 +3223,16 @@ gdk_window_add_damage (GdkWindow *toplevel,
 {
   GdkDisplay *display;
   GdkEvent event = { 0, };
+
+  /* This function only makes sense for offscreen windows. */
+  g_assert (gdk_window_is_offscreen (toplevel));
+
   event.expose.type = GDK_DAMAGE;
   event.expose.window = toplevel;
   event.expose.send_event = FALSE;
   event.expose.region = damaged_region;
   cairo_region_get_extents (event.expose.region, &event.expose.area);
+
   display = gdk_window_get_display (event.expose.window);
   _gdk_event_queue_append (display, gdk_event_copy (&event));
 }