
 <code>Map</code> interface over two logical maps.  The choice of
 which logical map is accessed depends on the current faces lifecycle
 phase.  One logical map is for the current traversal and the other is
 for the next traversal.  During the execute portion of the lifecycle,
 all flash accesses are sent to the current traversal map.  During the
 render portion of the lifecycle, all flash accesses are sent to the
 next traversal map.  On the next traversal through the lifecycle, the
 implementation must e