bug#19993: 25.0.50; Unicode fonts defective on Windows

[Eli Zaretskii]

> Date: Wed, 11 Mar 2015 12:49:39 -0700
> From: Ilya Zakharevich <ilya@math.berkeley.edu>
> Cc: 19993@debbugs.gnu.org
> 
> So the real question is not whether the presence of a Subset is used
> as filters, but: is
> 
>     the presence of the required character in the font
> 
> used as a filter.

When a font matches all the other constraints, then yes, it is
actually tested for whether it supports the specific character we need
to display.  See font_has_char and its callers.