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

[Eli Zaretskii]

> From: Andrey Orst <andreyorst@gmail.com>
> Date: Wed, 16 Oct 2019 15:08:53 +0300
> Cc: 37774@debbugs.gnu.org
> 
> > The assumption behind this feature was that the absolute majority of
> > faces don't need to be extended.  If you say this is wrong, can you
> > show enough examples to back up that?
> 
> I understand this, and maybe package maintainers should adopt the change

The assumption was that they won't need to adapt, because users will
want most faces should not to be extended.  If indeed many faces need
adaptation, then I do see a problem of backward compatibility; but
that wasn't the assumption.