Since we dropped the move region optimization there is really no need
to try carefully keep track of opaque non-overlapped regions, as we
don't use this information to trigger the optimization anymore.

So, by assuming that all windows are non-opaque we can vastly simplify
the clip region stuff. First of all, we don't need clip_region_with_children,
as each window will need to draw under all children anyway. Secondly, we
don't remove overlapping sibling areas from clip_region, as these are
all non-opaque anyway and we need to draw under them

Finally, we don't need to track the layered region anymore as its
essentially unused. The few times something like it is needed we can
compute it explicitly.

For the case of native children of widgets we may cause a repaint
under native windows that are guaranteed to be opaque, but these
will be clipped by the native child anyway.