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

[Gregory Heytings]

> > IIUC, what Stefan suggests is the following, which seems (almost) fine 
> > to me.
>
> I fail to see the difference, sorry.  Instead of doing the narrowing 
> from C we leave it to jit-lock, but tell it the limits to narrow to? Or 
> am I missing something?

TBH, I don't see a real difference either.  Stefan says that "this way 
it's easy to later refine the mechanism", I guess because it is possible 
to "refine the mechanism" without recompiling Emacs.  But as I said the 
problem is that there are other users of fontification-functions besides 
jit-lock-function.

Could you (or Stefan) please have a look at the bytecomp.el problem I 
mentioned at the end of my previous post?