if greater than 1, then the frame is obscured - we still consider
it to be "visible" as seen from lisp, but we don't bother
updating it. We must take care to garbage the frame when it
if greater than 1, then the frame is obscured - we still consider
it to be "visible" as seen from lisp, but we don't bother
updating it. We must take care to garbage the frame when it