window: Freeze stack when calculating showing state

Constantly manipulating the stack caused severe stalls (several seconds)
with many open windows when switching workspaces. The cause for this was
that each show/hide call dealt with the stack in isolation, meaning if
you hid N windows, we'd manipulate and synchronize the stack N times,
potentially doing synchronous calls to the X server while doing so.

Avoid the most severe stalls by freezing the stack while calculating
showing; this made the worst case go from several seconds to around
10-20 ms, which is still bad, but by far not as bad.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1616>


(cherry picked from commit d43c8cd8)