This never worked before csw since the root window is never
set as IS_MAPPED, but with the new viewable check (which is
true for the root window) we could erronously queue exposes
on the root window.

This happened unexpectedly in bug 589369, where metacity
got a GraphicsExpose event on the root window due to some
graphics operation, queueing an expose which would be handled
by clearing that area. That is fixed with this commit.