(check_it, init_from_display_pos, handle_stop)

(check_it, init_from_display_pos, handle_stop)
(setup_for_ellipsis, handle_single_display_spec)
(handle_composition_prop, next_overlay_string)
(get_overlay_strings, reseat_1, reseat_to_string)
(next_element_from_ellipsis, BUFFER_POS_REACHED_P)
(in_display_vector_p, display_line, get_next_display_element):
Change it->method from function pointer to enum.
(get_next_element): New array to map it->method to function.
(get_next_display_element): Use it.
(set_iterator_to_next): Use switch instead of if/else chain.