Improve font locking in buffers with long lines (bug#56682).

Improve font locking in buffers with long lines (bug#56682).

* src/dispextern.h (struct it): New 'narrowed_zv' field.

* src/xdisp.c (init_iterator): Set the field.
(get_narrowed_zv): New function to set the field.
(handle_fontified_prop): Use the field, together with 'narrowed_begv',
to narrow the portion of the buffer that 'Vfontification_functions'
will see.  Also bind 'inhibit-widen'.
(get_narrowed_len): New function, factored out of 'get_narrowed_begv'.
(unwind_narrowed_zv): New function.

* src/editfns.c (syms_of_editfns): New variable and symbol 'inhibit-widen'.
(Fwiden): Do nothing when 'inhibit-widen' is non-nil.