Improve Bidi with long lines.

Improve Bidi with long lines.

* src/composite.c (composition_compute_stop_pos): Use an 'endpos' that
is not too far away.
(find_automatic_composition): Use a 'head' that is not too far away.
Also make sure that this code path is not taken when long line
optimizations are disabled.

* src/dispextern.h (struct composition_it): Add a field that points
to the parent iterator.

* src/xdisp.c (init_iterator): Set it.