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

[Gregory Heytings]

> Gregory, is there a reason why the long-lines-improvements branch is not 
> yet merged?  I think it already includes important improvements that 
> should be exposed to a larger population for testing and feedback.

I was planning to add more improvements, but indeed merging that one and 
creating yet another one seems better at this point.

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?

In this case I did not find a good test case in the wild (or more 
precisely, I had no idea how to find one), so I concoted one myself. 
You'll find it here: https://www.heytings.org/data/locales.json .  Emacs 
is still a bit sluggish with that file, but behaves much better than 
before.

> If you for some reason prefer to keep that branch active, can you please 
> merge the current master to it, so that the changes in narrow-to-region 
> will be on the branch?

Is that feasible?  Non fast forwards are not allowed in the Emacs 
repository, so rebasing a feature branch is not possible (without using 
the workaround of deleting and recreating the branch).