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

[Gregory Heytings]

> > It is necessary to add a locked narrowing around 
> > fontification-functions and pre/post-command-hook to ensure that Emacs 
> > remains responsive.
>
> It's neither necessary (it's perfectly possible to do something quickly 
> that just needs to look at the first few lines of the buffer to decide 
> in which way to parse the nearby surrounding bytes, for example) nor 
> sufficient (it's easy to spend minutes wasting time running in circles 
> because of a bug, e.g. a bug triggered by the fact that the "unusual" 
> nature of the visible part of the buffer after an arbitrary narrowing).

You're a mathematician, aren't you?  I'm not speaking of mathematical 
(i.e. absolute) necessity here, but of a practical necessity.  And I did 
not say (or thought) that it is sufficient.