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

[Gregory Heytings]

> > navigating through the locales.json file was slow (at some positions) 
> > before the change and is now reasonably fast (but alas not 
> > instantaneous).
>
> Which navigation commands were slow, as compared to the same commands in 
> other portions of this file?

For example C-p, C-n, C-v, M-v.  C-f and C-b also, but much less so.

For example, open the file and do M-g c 70000 RET.  This took about 5 
seconds.  Now do C-n, this again took about 5 seconds.  With the 
optimizations, M-g c 70000 RET is almost immediate, and C-n there takes 
less than a second.

But you're right, this slowdown has little to do with bidi.  A file with a 
sufficiently long single line of Arabic text has the same problem.  (But 
not one with a line of Hebrew text.)