bug#43177: Bug: Emacs 27.1 hangs forever in `FcCharSetSubtractCount' from '/usr/lib/libfontconfig.so.1'

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: schwab@linux-m68k.org,  43177@debbugs.gnu.org,
>   emacs@Alexander.Shukaev.name
> Date: Fri, 04 Sep 2020 15:05:52 +0200
> 
> +          {
> +            list = Fcons (val, list);
> +            /* Querying further backends can be very slow, so we only do
> +               it if the user has explicitly requested it (Bug#43177).  */
> +            if (query_all_font_backends == false)
> +              break;
> +          }

Shouldn't we query those other backends if the first one couldn't find
a font, even if the variable is nil?  Otherwise, this change could
cause regressions in some (hopefully rare) cases, whereby some
characters will display as hex codes where previously they were shown
using some font.