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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56682@debbugs.gnu.org
> Date: Tue, 26 Jul 2022 17:17:29 -0400
> 
> > Allowing fontification-functions to search for arbitrarily complex
> > regexpes in an arbitrarily large buffer, each and every time they are
> > asked to to highlight a small chunk of the said buffer, is a recipe
> > for disaster.
> 
> `font-lock.el` could enforce a smaller scope in a more discerning way
> that narrowing can.
> 
> > If for some reason modes really need to go through to the whole
> > buffer to decide which highlighting to use, they should to do so outside of
> > fontification-functions, and ideally once, for example, when the file
> > is loaded.
> 
> With the current narrowing they can't even know why the buffer is
> narrowed and hence can't make an informed decision whether they should
> maybe widen to look elsewhere.

Feel free to suggest better ways of handling these issues, or even
ways to solve this entirely inside font-lock.  If and when such
suggestions materialize, I'm sure we will be glad to use them instead
of less elegant/more direct solutions.