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

[Gregory Heytings]

> > At least if I do (pos-visible-in-window-p (point-max)), they are not: 
> > handle_fontified_prop is not even called with it at point-max. Even 
> > with (pos-visible-in-window-p (1+ (window-end))) 
> > fontification-functions are not called.
>
> Try with vertical-motion.  Visit long-line.xml, go to position 20000, 
> and then do "C-u 200 C-n" or "M-: (vertical-motion 200) RET.  Sooner or 
> later you will see that it->current in handle_fontified_prop will be 
> outside of the narrowing.

Thanks, I was able to reproduce the bug with that recipe.

> > Should we perhaps be extra careful and add not apply the narrowing when 
> > IT_CHARPOS is not between narrowed_begv and narrowed_zv?
>
> I'd rather narrow around IT_CHARPOS in that case.  That would be also 
> consistent with what the doc string of fontification-functions now says.
>
> Perhaps we should also change what init_iterator does: if the start 
> position with which it's called is outside of the restriction, recompute 
> the restriction using the start point instead of the window's point 
> position.  WDYT?

Doing it in init_iterator is too early alas, with the above recipe at 
least init_iterator is called with charpos inside the narrowing bounds, 
after which the iterator moves outside the narrowing bounds.  So I fixed 
the bug in handle_fontified_prop.

I don't know yet if it's necessary to add another similar recomputation 
inside init_iterator.