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

[Gregory Heytings]

> The alternative, of course, is to pay the price of syntax-ppss on larger 
> spans and wait the corresponding amount of time the first time the user 
> scrolls to EOB.

Or to speed up syntax-ppss in a way or another.

> But as Gregory shows, when you get to _really_ large files (like 1 GB 
> JSON file in his example), pressing M-> will still make you wait (I have 
> to wait around 20 seconds).

Or when you have a slower CPU, with moderately large files.

> But being able to choose this approach (in the absence of better 
> upcoming alternatives) is a good thing.

The problem is (again) that what you would like does not provide any 
protection against modes which use widen inside their fontification 
routines.