Speed up cursor motion in large fontified buffers.

Speed up cursor motion in large fontified buffers.

 src/xdisp.c (reseat): Don't look for prev_stop, as that could mean a
 very long run.
 (next_element_from_buffer): When iterator oversteps prev_pos
 backwards, don't search for a new prev_stop more than 1000
 characters back.
 (handle_stop_backwards): Don't assume that CHARPOS is necessarily
 a stop_pos.
 (compute_display_string_pos): Check also BUF_OVERLAY_MODIFF for a
 match, when testing the cached display string position for
 applicability.