Canvas: fix gtk 3.9 redraw issues.

From 735adfe94dc5b30a8d0c75385a9749937c3a0ab4 Mon Sep 17 00:00:00 2001
From: Alexander Larsson <alexl@redhat.com>
Date: Tue, 20 Aug 2013 13:14:58 +0200
Subject: [PATCH] canvas: Look at cairo clip region rather than current expose
 event

This fixes redraw issues with latest Gtk+ which only sends expose events
for the actual toplevel windows rather than each GdkWindow.