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,
>   visuweshm@gmail.com
> Date: Sat, 23 Jul 2022 19:23:40 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Font-lock Scroll-bar Composition Output
> >> ------------------------------------------------------------
> >> on   on         on          GCs: 14 Elapsed time: 7.276626 seconds
> >> on   off        on          GCs: 1 Elapsed time: 5.363002 seconds
> >> on   off        off         GCs: 1 Elapsed time: 3.967520 seconds
> >
> > Thanks.  How about now, after I installed the last change?
> 
> 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?