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.

> So I would recommend against trying to solve this part right now.

It doesn't only solve the syntax-ppss problem, it also makes flyspell-mode 
usable in such files, for example.