For clipping the shadow buffer, we should use the same trick like in https://bugs.webkit.org/show_bug.cgi?id=43416 to speed it up when using Qt >= 4.8.
Created attachment 65011 [details] Patch
Comment on attachment 65011 [details] Patch LGTM
Committed r65776: <http://trac.webkit.org/changeset/65776>