Avoid needlessly re-compositing opaque windows in front of other damaged...

Avoid needlessly re-compositing opaque windows in front of other damaged windows. This also helps with stuff that isn't properly redirected yet, such as Xv and DRI with free drivers (patch originally contributed by Michel Dänzer <michel@tungstengraphics.com>).
Fix compilation with --enable-debug=full.


(Old svn revision: 22034)