Fix a crash that was happening if we had 2 exported canvases in a .sif file where...

Fix a crash that was happening if we had 2 exported canvases in a .sif file where the 2nd contained a pastecanvas that used the 1st as its canvas.  The canvases are destructed in the order in which they appear in the .sif file, so by the time the 2nd is destroyed, the pastecanvas it contains no longer has a valid canvas parameter.  The fix is to clear the canvas parameter of any pastecanvases using a canvas when the canvas itself is destroyed.

git-svn-id: http://svn.voria.com/code@1541 1f10aa63-cdf2-0310-b900-c93c546f37ac