Speed up keyboard auto-repeat cursor motion under bidi redisplay.

Speed up keyboard auto-repeat cursor motion under bidi redisplay.

 src/xdisp.c (compute_stop_pos_backwards): New function.
 (next_element_from_buffer): Call compute_stop_pos_backwards to
 find a suitable prev_stop when we find ourselves before
 base_level_stop.
 (reseat): Don't look for prev_stop, as that could mean a very long
 run.
 <cached_disp_pos, cached_disp_buffer, cached_disp_modiff>
 <cached_disp_overlay_modiff>: Cache for last found display string
 position.
 (compute_display_string_pos): Return the cached position if asked
 about the same buffer in the same area of character positions, and
 the buffer wasn't changed since the time the display string
 position was cached.