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

[Gregory Heytings]

> > No, it happens when the buffer is opened.  Given the importance that 
> > you and Stefan seem to give to that function, it is, with the patch I 
> > sent in my previous post, called once on the whole buffer (without any 
> > narrowing) when the file is opened.
>
> But if the buffer is not scrolled to the end, shouldn't it be called 
> with a position that's close to the beginning?
>
> That shouldn't force the full buffer scan, meaning this call should 
> complete quickly.

I'm trying to reconcile two conflicting constraints.

It is necessary to add a locked narrowing around fontification-functions 
and pre/post-command-hook to ensure that Emacs remains responsive.

At the same time, you and Stefan tell me that syntax-ppss does an 
important job and will not do it correctly with such a locked narrowing, 
IOW, that at least syntax-ppss should be called without a locked 
narrowing.  But you also tell me that its result is cached so that a full 
buffer scan isn't necessary anymore when it has happened at least once.

So what I'm suggesting is to do a full buffer scan immediately, when the 
file is opened, without any narrowing.  If that happens, later calls to 
syntax-ppss inside fontification-functions and pre/post-command-hook will 
use the cached result of the initial scan, and will do their job correctly 
even with a locked narrowing.

Unless I misunderstand something, I think (and my tests seem to confirm) 
that that would be a workable solution, provided that the initial scan is 
reasonably fast, that is, at least six times faster than it is now.