(handle_face_prop): Test for strings that came from overlays,

(handle_face_prop): Test for strings that came from overlays,
not just for after-strings and before-strings.
Call face_for_overlay_string and pass the overlay to it.
(handle_display_prop): Determine whether property came from an overlay.
Pass OVERLAY arg to handle_single_display_spec.
(handle_single_display_spec): New arg OVERLAY sets it->from_overlay.
(load_overlay_strings): Fill in it->string_overlays.
(get_overlay_strings_1, push_it, pop_it): Handle it->from_overlays.