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

[Gregory Heytings]

> > I pushed another improvement to the branch.  Now motion in a 
> > font-locked buffer is almost as fast as in a non font-locked one; I 
> > don't think it's possible to make further significant improvements.
>
> Font-lock Scroll-bar Composition Output
> ------------------------------------------------------------
> on        on         on          GCs: 13 Elapsed time: 1.632215 seconds
> on        off        on          GCs: 1 Elapsed time: 1.433515 seconds
> on        off        off         GCs: 2 Elapsed time: 1.209662 seconds
> off       off        off         GCs: 2 Elapsed time: 0.524913 seconds

Thanks, this confirms my "almost as fast" feeling and measurements.