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

[Gregory Heytings]

> > But one big slow scan (and how slow it is actually depends on a 
> > particular major mode) followed by responsive editing sounds much 
> > better than what we've had before.
>
> Indeed.  But then the question is: is it possible to do that scan while 
> opening the file, before it becomes editable?  It is way better to wait 
> a few seconds more while the file is being opened than to wait before 
> two basic motion commands when the file is already opened.

Sorry, I meant "to wait _between_ two basic motion commands".