Performance improvement in find_cwindow_in_screen
On my system, 2-4% of a CPU core from xfwm4 is being spend in find_cwindow_in_screen pulling in entries from the linked list on a lightly updating desktop. After the attached patch, 25-50% of that CPU is gone, presumably because the slice allocators have better locality of the list-entries, resulting in a 1-2% total drop in CPU usage from xfwm4 for this workload.