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

[Gregory Heytings]

> > > > > There have been many more complaints about being unable to edit a 
> > > > > file with a 5000+ line comfortably, than about editing 100+ MB 
> > > > > files.
> > > >
> > > > Also because we have things like so-long to handle the large file 
> > > > case.
> > >
> > > These things are expedients for want of anything better, aren't they?
> >
> > Yes, just like the narrowing your use in your long lines patch.
>
> That's... unfair.  These things turn off everything (in particular, the 
> major mode and font locking) and provide a bare-bones editing 
> experience. What we'd like now is to provide Emacs users a near-complete 
> editing experience in all buffers.

And, by the way, what could be a better solution?  Without assuming that 
computers have infinitely fast CPUs, that is?