Fix previous change that broke faces in bidirectional text.

Fix previous change that broke faces in bidirectional text.

 src/xdisp.c (compute_stop_pos_backwards): New function.
 (handle_stop_backwards): Revert last change.
 (next_element_from_buffer): Call compute_stop_pos_backwards to
 find a suitable prev_stop when we find ourselves before
 base_level_stop.  Remove the funky search for 1000 character
 positions back.