(next_overlay_string): Fix assert.

(next_overlay_string): Fix assert.
(push_it, pop_it): Handle composition and stretch specific values.
Only handle it->slice in image (for now).
(back_to_previous_visible_line_start): Continue search if newline is
part of a compisition.  Simplify.
(reseat_1): Set it->object to buffer.
(set_iterator_to_next): Set it->object to string or buffer, when
setting it->method to GET_FROM_STRING or GET_FROM_BUFFER.
(next_element_from_composition): Set it->object to buffer if not
from string.
(set_cursor_from_row): Only save start of string if not already
done to handle multiple strings in a row.