bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> Date: Sat, 30 Jul 2022 13:31:42 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, 56682@debbugs.gnu.org, larsi@gnus.org, 
>     monnier@iro.umontreal.ca
> 
> So the only remaining question is whether it is necessary to recompute 
> narrowed_begv and narrowed_zv in init_iterator:

I tend to think we should, but let me think about this some more.