We don't track the full clip for each window anymore, as this
is not useful when no windows are opaque. However, we still
need the full clip for the shape, so its calculated manually.

However, it was previously only recalculated when the clip changes
which doesn't correctly handle the case of a sibling geometry changing.

So, instead of doing this directly when geometry changes we just
set a bit in the toplevel whenever some window geometry changes, and
we then handle this in process_updates, updating the shape for all
native windows. This should be ok performance-wise because we don't
expect a lot of native children.