Save and restore bidi cache when saving and restoring the iterator.

Save and restore bidi cache when saving and restoring the iterator.
Not tested, just compiled.

 src/bidi.c (bidi_shelve_cache, bidi_unshelve_cache): New functions.
 src/dispextern.h (bidi_shelve_cache, bidi_unshelve_cache): Declare
 prototypes.
 src/xdisp.c (SAVE_IT, RESTORE_IT): New macros.
 (pos_visible_p, face_before_or_after_it_pos)
 (back_to_previous_visible_line_start)
 (move_it_in_display_line_to, move_it_in_display_line)
 (move_it_to, move_it_vertically_backward, move_it_by_lines)
 (try_scrolling, redisplay_window, display_line): Use them when
 saving a temporary copy of the iterator and restoring it back.