Fix 1826376, 1848018, 1901753. All caused by not making a copy of a canvas before...

Fix 1826376, 1848018, 1901753.  All caused by not making a copy of a canvas before exporting it.  This fix isn't ideal because in some cases it isn't necessary to duplicate the canvas, and it also leaves the canvas parameter of any pastecanvas layers which use the canvas with their original value until it is changed manually, but it stops the crashes.

git-svn-id: https://synfig.svn.sourceforge.net/svnroot/synfig@2216 1f10aa63-cdf2-0310-b900-c93c546f37ac