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

[Stefan Monnier]

> And even if one sets font-lock-dont-widen to nil, mode authors are
> completely free to ignore that setting (assuming they're aware it exists).

Hmm... `font-lock-dont-widen` is not about telling the major modes's
font-lock code to refrain from narrowing.  It's there so a major mode
(like mmm-mode) can tell font-lock.el to refrain from widening.

The major-mode's own font-lock rules should never need to widen
(because font-lock itself does it, normally).  If they do, it's a bug.


        Stefan