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

[Gregory Heytings]

> So there is a particular performance problem with the display of 
> fontified buffers which I'd really like your help in fixing.

Thank you, but that problem has, as you can see, already been fixed.

> Fixing in a way that doesn't add narrowing around 
> fontification-functions, because as we can see it's not necessary in 
> examples like this.

As I said, your 88K example file is not representative.  So the fact that 
it's not necessary in that tiny example file says nothing about much 
larger files.

> I think we could have both speed and correctness, at least for files of 
> this size.

Feel free to enlarge long-line-threshold in your init file.