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

[Stefan Monnier]

>> So ideally font-lock is either called with undo-able narrowing, or is 
>> simply passed a range of positions, and shouldn't fontify too far from them.
> Many major-modes do widen the buffer, though.

Actually, IIUC this should be considered a bug (it breaks the use of
that major in mmm-mode and friends).

[ And `grep` suggests that less than half of progmodes/*.el use `widen`.  ]


        Stefan