bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Wed, 16 Oct 2019 21:18:06 +0300
> Cc: 37774@debbugs.gnu.org
> 
> The main task is to get background colors back to extend to the
> window edge.

If that's the main task, then the easiest way is just to revert all
the changes that introduced that feature.

(It's a pity that the long discussion of this before the development
started went without any such objections from the people who are
regulars on emacs-devel.)