gdkwindow: plug memory leak

gdkwindow: plug memory leak

==23282== 64 bytes in 2 blocks are definitely lost in loss record 8,069 of 13,389
==23282==    at 0x4A074CD: malloc (vg_replace_malloc.c:236)
==23282==    by 0x39A1C3E2EA: cairo_region_create (cairo-region.c:196)
==23282==    by 0x6D9AF3D: recompute_visible_regions_internal (gdkwindow.c:964)
==23282==    by 0x6D9B4B8: recompute_visible_regions (gdkwindow.c:1126)
==23282==    by 0x6DA3450: gdk_window_hide (gdkwindow.c:5689)
==23282==    by 0x6D9CED9: _gdk_window_destroy_hierarchy (gdkwindow.c:2042)
==23282==    by 0x6D9D040: gdk_window_destroy (gdkwindow.c:2109)
==23282==    by 0x655B5E4: gtk_entry_unrealize (gtkentry.c:3012)
==23282==    by 0x7068BF3: g_cclosure_marshal_VOID__VOID (gmarshal.c:85)
==23282==    by 0x706710B: g_type_class_meta_marshal (gclosure.c:885)
==23282==    by 0x7066DF9: g_closure_invoke (gclosure.c:774)
==23282==    by 0x7080585: signal_emit_unlocked_R (gsignal.c:3340)
==23282==    by 0x707F619: g_signal_emit_valist (gsignal.c:3033)
==23282==    by 0x707FB71: g_signal_emit (gsignal.c:3090)
==23282==    by 0x679E243: gtk_widget_unrealize (gtkwidget.c:4458)
==23282==    by 0x64E83C7: gtk_bin_forall (gtkbin.c:172)
==23282==    by 0x6548BBD: gtk_container_forall (gtkcontainer.c:2014)
==23282==    by 0x67A966D: gtk_widget_real_unrealize (gtkwidget.c:10253)
==23282==    by 0x672D002: gtk_tool_item_unrealize (gtktoolitem.c:474)
==23282==    by 0x7068BF3: g_cclosure_marshal_VOID__VOID (gmarshal.c:85)

https://bugzilla.gnome.org/show_bug.cgi?id=666552