kwin: fixing blur bug for move/resize events

The old blur version wrongly marked a cache region as valid with the
reason that this region would never become visible. It didnt matter
because the only case that this region could become visible was a
movement of the window on top, which back then would have forced a
workspace repaint of that region and as such would have invalidated
the cache anyway. With the introduction of addLayerRepaint the
last point is no longer true and we have to track the valid cache
regions more carefully.