In meta_screen_manage_all_windows() we can use our own stack
tracker to get the list of windows - no need to query X again.

A copy is needed because the stack gets modified as part of the loop.
Specifically, meta_stack_tracker_get_stack() at this time returns the
predicted stack, and meta_window_new() performs a few operations
(e.g. framing) which cause immediate changes to the predicted stack.

https://bugzilla.gnome.org/show_bug.cgi?id=721345