Also implements a new menu item flag: MENUITEMFLAG_SLIDER_DEFERRED,
which only calls the MENUOP_GETSLIDER callback on entering dimmed mode,
storing its value for later ticks, and only calls the MENUOP_SET
callback on leaving dimmed mode. This was necessary to prevent the
framerate limit slider from immediately affecting the framerate, leading
to an annoying slow down as the value was decremented to 0.