(handle_single_display_prop): Handle left-fringe and

(handle_single_display_prop): Handle left-fringe and
right-fringe similar to a display margin image.  Specifically,
the characters having the fringe prop are no longer shown, and
we use IT_IMAGE/next_element_from_image with image_id = -1 to
do this.  Set fringe bitmap face_id in it->face_id.
(produce_image_glyph): Handle image_id < 0 as "no image" case, but
still realize it->face (i.e. the fringe bitmap face).