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

[Dmitry Gutov]

On 26.07.2022 14:45, Eli Zaretskii wrote:
> > Perhaps it's due to font-lock logic in that it has to match from the
> > beginning of a line (not sure we'd want to abandon that promise,
> > though). Or maybe something else.
> It isn't font-lock, at least not in all major modes.  It's the display
> engine itself that sometimes needs to go to the beginning of the line.
> When it does, going back gets slower with font-lock than without.
> This is why you see slower redisplay when you go deeper into a long
> line.

Makese sense: the text only has to be fontified once, but all redisplays 
are slowed down (on master), not just the first one.