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

[Gregory Heytings]

The first step of the font locking related improvements was easier than I 
thought.

Two following commands:

1. C-x C-f dictionary.json RET y

2. C-e

on my laptop, with Emacs from master from a week ago, take respectively 
150 seconds and 40 seconds.  With the improvements on master, 1 is 
instantaneous but 2 still takes about 5 seconds.  Now, with the changes in 
the feature/long-lines-and-font-locking branch, both are instantaneous. 
The price of that speedup is that some portions of the buffer will be 
mis-highlighted, which is unavoidable.