Improve narrowing when iterator has moved outside of narrowing bounds.

Improve narrowing when iterator has moved outside of narrowing bounds.

* src/xdisp.c (get_narrowed_begv, get_narrowed_zv): Add 'pos'
parameter.
(init_iterator): Add arguments to 'get_narrowed_begv' and
'get_narrowed_zv'.
(handle_fontified_prop): Recompute the narrowing when iterator
has moved outside of narrowing bounds.

* src/dispextern.h (get_narrowed_begv, get_narrowed_zv): Adapt
prototypes.

* src/composite.c (find_automatic_composition): Add argument
to 'get_narrowed_begv'.