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

[Gregory Heytings]

> > If the conclusion is, after some reasonable effort, that there is no 
> > way to make syntax-ppss significantly faster in one way or another in 
> > such cases, and that there is no way to make font locking reasonably 
> > accurate even when it doesn't have access to the whole buffer, it might 
> > make sense to provide user options to fine-tune the behavior.  But we 
> > are not there yet.
>
> Both conclusions lead to removing the applications of narrowing from 
> handle_fontified_prop.

I have seen no effort whatsoever to make syntax-ppss significantly faster 
in one way or another.