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

[Gregory Heytings]

> I don't think major modes would take advantage of that var. I hope not, 
> at least.

You're optimistic, let's say I'm less optimistic.

> If you wanted the development to take this route, it would have really 
> made more sense to hold off on applying the narrowing to 
> fontification-functions, keeping font-lock slow-ish in large files. That 
> would have encouraged direct work on speeding it up.

That's your opinion.  What I think would have most likely happened is what 
you yourself said: these are large files, it's unavoidable that font 
locking takes somes time (these are not your exact words, I hope they 
express what you said faithfully).

> But anyway. Suppose we have some more success with syntax-ppss. Two 
> options:

There are many other options besides the two options you listed.