Fix list traversal issue in client_calc_layer

Fix list traversal issue in client_calc_layer

The calls to client_calc_layer_internal can modify stacking_list, which
can cause us to follow dangling ->next pointers (either by the pointer
itself already being freed, or it pointing to a freed area). Avoid this
by copying the list first, the goal is to visit every client in the list
once so this should be fine.