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

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: 56682@debbugs.gnu.org,  gregory@heytings.org,  monnier@iro.umontreal.ca
> Date: Sat, 23 Jul 2022 17:04:39 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Perhaps when
> > long-line optimizations are turned on for a buffer, we shouldn't try
> > to be holier than the Pope, and should use window_end_pos even if not
> > reliable?
> 
> I don't think anyone would complain.

I agree, and so I've now done so on the branch.