(handle_stop): Initialize it->add_overlay_start to zero.

(handle_stop): Initialize it->add_overlay_start to zero.
(handle_invisible_prop): Record the start of invisible text in
it->add_overlay_start.
(struct overlay_entry): Add member `overlay'.
(handle_overlay_change): Simplify.
(next_overlay_string): After having processed overlay strings at the
end of the buffer, record that fact in
it->overlay_strings_at_end_processed_p.
(compare_overlay_entries): If before- and after-strings come
from the same overlay, let before-strings come first.
(RECORD_OVERLAY_STRING): Record the overlay that strings come from.
(load_overlay_strings): Take it->add_overlay_start into account
when adding overlay strings.