Support bidi reordering of unibyte strings. Fix crash displaying "All" in mode line of an empty buffer.
src/dispextern.h (struct bidi_string_data): New member `unibyte'.
src/xdisp.c (handle_single_display_spec, next_overlay_string)
(get_overlay_strings_1, reseat_1, reseat_to_string)
(push_display_prop): Set up the `unibyte' member of bidi_it.string
correctly. Don't assume unibyte strings are not bidi-reordered.
(compute_display_string_pos)
(compute_display_string_end): Fix handling the case of C string.
src/bidi.c (bidi_count_bytes, bidi_char_at_pos): Accept an
additional argument UNIBYTE, and support unibyte strings. All
callers changed.
(bidi_fetch_char): Support unibyte strings.