fix for snapshot module if canvas exceeds thumbnailsize

Hi, this is the fix from this mail:
From: Wolfgang Goetz <Wolfgang.ztoeG@web.de>
To: darktable-devel@lists.sourceforge.net <darktable-devel@lists.sourceforge.net>
Subject: [darktable-devel] snapshotting module for big drawing areas... fixed
Date: Fri, 26 Jul 2013 23:51:49 +0200