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

[Gregory Heytings]

> So being able to fine-tune the behavior makes sense. Certainly not a 
> "maintenance nightmare".

If the conclusion is, after some reasonable effort, that there is no way 
to make syntax-ppss significantly faster in one way or another in such 
cases, and that there is no way to make font locking reasonably accurate 
even when it doesn't have access to the whole buffer, it might make sense 
to provide user options to fine-tune the behavior.  But we are not there 
yet.