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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: gregory@heytings.org,  dgutov@yandex.ru,  56682@debbugs.gnu.org
> Date: Wed, 03 Aug 2022 16:36:05 -0400
> 
> > What do you mean by "accidentally" in this context?  A program rarely
> > does anything "accidentally", it almost always does what the
> > programmer wanted it to do.
> 
> I mean that currently font-lock just blindly widens all the time, so we
> should fix it so it only widens when it's expected to (i.e. not when
> the widening is installed by the LLT code).

So let's please fix that, and take it from there.  We have plenty of
time to make followup decisions after we see what that does.