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

[Stefan Monnier]

> What do you mean by "accidentally" in this context?  A program rarely
> does anything "accidentally", it almost always does what the
> programmer wanted it to do.

I mean that currently font-lock just blindly widens all the time, so we
should fix it so it only widens when it's expected to (i.e. not when
the widening is installed by the LLT code).


        Stefan