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

[Gregory Heytings]

> > I just pushed an improvement for Bidi.  I did not do so earlier because 
> > I was trying to improve it further, but somehow I'm hitting a brick 
> > wall here.  Could you please check, and tell me if what I already did 
> > is okay?
>
> Hmm... which part(s) of the recent commit(s) on the branch are related 
> to bidi?  I only see changes related to composed characters.  What did I 
> miss?

82b602dc2f improves bidi in long lines, without indeed touching bidi.c.