bug#33442: 26.1.90; Cairo side-by-side windows scrolling fix should be cherry-picked

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Thu, 29 Nov 2018 14:19:56 +0100
> Cc: 33442@debbugs.gnu.org, Dmitry Gutov <dgutov@yandex.ru>
> 
> > With the following patch, GDK_SCALE=2 seems to work for me.
> >
> 
> Works for me. Since this is all Cairo-only code, it could even go into
> emacs-26, I think (with a ChangeLog and perhaps some comments).

OK, let's do that.