[quadrapassel] Fix window size increase under GTK+ 3.20

[Mario Wenzel <mariowenzel src gnome org>]

commit 1593e5826d3916bc25f6b2cd7e6218c000fe542d
Author: Mario Wenzel <maweki gmail com>
Date:   Mon Feb 15 17:38:11 2016 +0100

    Fix window size increase under GTK+ 3.20

 src/quadrapassel.vala |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/src/quadrapassel.vala b/src/quadrapassel.vala
index a7258a3..22d1810 100644
--- a/src/quadrapassel.vala
+++ b/src/quadrapassel.vala
@@ -207,8 +207,7 @@ public class Quadrapassel : Gtk.Application
     {
         if (is_maximized || is_tiled)
             return;
-        window_width = allocation.width;
-        window_height = allocation.height;
+        window.get_size (out window_width, out window_height);
     }
 
     private bool window_state_event_cb (Gdk.EventWindowState event)