The PaintRedirector calls the new method KDecoration::render and passes
it's PaintDevice and the region to update to it. A decoration can
implement this method and provide an optimized implementation for the
painting which does not go through the deco's QWidget at all. In addition
the decoration can invoke an update() slot which will schedule a repaint
in the PaintRedirector and thus completely replaces the need for
intercepting paint events on the QWidget and also allows to add QWindow
based decorations in future.