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

[Eli Zaretskii]

> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca,
>  visuweshm@gmail.com
> Date: Sat, 23 Jul 2022 20:44:43 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > Font-lock Scroll-bar Composition Output
> > ------------------------------------------------------------
> > on    on         on          GCs: 16 Elapsed time: 5.496764 seconds
> > on    off        on          GCs: 1 Elapsed time: 5.362916 seconds
> > on    off        off         GCs: 1 Elapsed time: 3.947306 seconds
> > 
> > That's with
> > 280b8c96cc origin/feature/long-lines-and-font-locking Improve display of 
> > columns on mode-lin
> 
> Thanks.  So the scroll-bar effect is largely gone, and font-lock is
> now just 25% slower than no-font-lock.  Which I think is reasonable,
> given that there's a face change every 10 to 20 characters?

Oops, I was confused: font-lock OFF is not in the table, so with it
we're still 4 to 5 times slower than without it.  It's
auto-composition mode that costs us 25% slowdown.