bug#22404: 25.1.50; Forcing `window-scroll-functions` to run.

[Eli Zaretskii]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: Keith David Bershatsky <esq@lawlist.com>,  22404@debbugs.gnu.org
> Date: Fri, 29 Jan 2016 13:00:42 +0100
> 
> Anyway, using pre-command-hook is too late for us: it's not executed
> unless the next input arrives, so you have to hit a key until
> decorations or whatever get updated.

Then maybe we need a post-redisplay-hook.