Fix excessive calls to bidi_shelve_cache reported in bug #15555.

Fix excessive calls to bidi_shelve_cache reported in bug #15555.

 src/xdisp.c (move_it_in_display_line_to): Save the iterator state in ppos_it
 only once per call.  Reimplement the method used to return to the
 best candidate position if all the positions found in display line
 are beyond TO_CHARPOS.  This cuts down the number of calls to
 bidi_shelve_cache, which moves a lot of stuff when lines are long
 and include bidirectional text.