Fix bug #9218 with slow cursor motion and scrolling Org Mode buffers.

Fix bug #9218 with slow cursor motion and scrolling Org Mode buffers.

 src/dispextern.h (struct bidi_it): New member disp_prop_p.
 src/xdisp.c: Remove one-slot cache of display string positions.
 (compute_display_string_pos): Accept an additional argument
 DISP_PROP_P; callers changed. Scan at most 5K characters forward
 for a display string or property.  If found, set DISP_PROP_P
 non-zero.
 src/bidi.c (bidi_fetch_char): Accept an additional argument
 DISP_PROP_P, and pass it to compute_display_string_pos.  Only
 handle text covered by a display string if DISP_PROP_P is returned
 non-zero.  All callers of bidi_fetch_char changed.