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

[Gregory Heytings]

> > Indeed, that seems to be the culprit here, I didn't know that text-mode 
> > was an exception here.  If I set bidi-paragraph-direction to 
> > 'left-to-right after visiting the arabic-small.txt file, Emacs (mis) 
> > behaves like it does for the other Arabic files: it becomes slow, and 
> > C-n C-p do not work correctly anymore.
>
> I've installed a change that makes Arabic rendering roughly twice 
> faster.  It is still quite slow, but less so.

Yes, I've seen that change, too.  It's better now, I wonder if I'll manage 
to make it even better.