gdk: Ensure flush of surface when cairo_t is destroyed

Add an ugly workaround because GTK does not ensure surfaces get flushed
before directly accessing the drawable backed by the surface. This is
not visible on X11 (where flushing is a no-op), but can be seen on
Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=628291

gdk/gdkcairo.c

@@ -23,6 +23,13 @@
 #include "gdkregion-generic.h"
 #include "gdkalias.h"
 
+static void
+gdk_ensure_surface_flush (gpointer surface)
+{
+  cairo_surface_flush (surface);
+  cairo_surface_destroy (surface);
+}
+
 /**
  * gdk_cairo_create:
  * @drawable: a #GdkDrawable
@@ -43,6 +50,7 @@
 cairo_t *
 gdk_cairo_create (GdkDrawable *drawable)
 {
+  static const cairo_user_data_key_t key;
   cairo_surface_t *surface;
   cairo_t *cr;
     
@@ -54,7 +62,12 @@ gdk_cairo_create (GdkDrawable *drawable)
   if (GDK_DRAWABLE_GET_CLASS (drawable)->set_cairo_clip)
     GDK_DRAWABLE_GET_CLASS (drawable)->set_cairo_clip (drawable, cr);
     
-  cairo_surface_destroy (surface);
+  /* Ugly workaround for GTK not ensuring to flush surfaces before
+   * directly accessing the drawable backed by the surface. Not visible
+   * on X11 (where flushing is a no-op). For details, see
+   * https://bugzilla.gnome.org/show_bug.cgi?id=628291
+   */
+  cairo_set_user_data (cr, &key, surface, gdk_ensure_surface_flush);
 
   return cr;
 }