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

[Gregory Heytings]

> The changes are related to compositions, not to bidi.  Displaying Arabic 
> (and maybe also other characters in that file) requires character 
> composition, but it has nothing in particular to do with bidi per se.

I know next to nothing about bidi, so it's very well possible indeed that 
I confused "bidi" and "composition" (or "bidi composition"?).  Anyway, 
navigating through the locales.json file was slow (at some positions) 
before the change and is now reasonably fast (but alas not instantaneous).

> Are you saying that if you replace the Arabic text there with some other 
> script that also requires composition processing (like one of the Indic 
> scripts, see lisp/language/indian.el), editing this file is 
> significantly faster?

I think the locales.json file contains samples of pretty much all 
available scripts.  Devanagari for example is around position 3260000. 
As far as I can tell, navigating in that part of the file is not 
significantly faster with the change.  It is only in the parts of the file 
that contain e.g. Arabic text that the speedup is visible, around position 
70000 for example.

> Btw, I'm unable to edit that file on the branch, because 
> show-paren--default causes an assertion violation.  I'm pretty sure 
> that's due to the issues in narrow-to-region that were already fixed on 
> master.

I just merged master into the feature branch.