bug#19117: 25.0.50; emacs on x11 chooses different fonts for the same face sometimes

[Eli Zaretskii]

> From: Dima Kogan <dima@secretsauce.net>
> Date: Tue, 30 Dec 2014 10:33:17 -0800
> Cc: 19117@debbugs.gnu.org
> 
> I will patch xfont_list_pattern() to accept a 3rd type of
> scalable-fonts-allowed: 'fallback where we add these fonts only if no
> others are available, and in realize_basic_faces() I'll use THAT value.
> Does that sound reasonable?

Please note that on MS-Windows the scalable fonts were always used in
this scenario, back with that code, and there was a comment explaining
that otherwise too few fonts will be available on Windows.