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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  56682@debbugs.gnu.org,
>   gregory@heytings.org
> Date: Tue, 02 Aug 2022 17:46:49 -0400
> 
> >> So ideally font-lock is either called with undo-able narrowing, or is 
> >> simply passed a range of positions, and shouldn't fontify too far from 
> >> them.
> > Many major-modes do widen the buffer, though.
> 
> Actually, IIUC this should be considered a bug (it breaks the use of
> that major in mmm-mode and friends).
> 
> [ And `grep` suggests that less than half of progmodes/*.el use `widen`.  ]

That fits my definition of "many".

And we also need to consider specifically those modes which are likely
to happen in files with long lines.