Fontique font enumeration is slow on some fontconfig systems

[tomcur]

NixOS is a specific example of a distribution where enumeration is slow (see also linebender/xilem#804). NixOS likely has a somewhat different fontconfig structure than other distributions, but I haven't looked in detail. I am on NixOS as well, and have noticed the same issue.

cc @kmicklas, @Philipp-M

[brynedwards]

Hello, for what it's worth I experienced this slowdown on NixOS (~2 seconds until window appeared) but #378 removed it.

[DJMcNab]

I'm going to close this then. We can always reopen if that's incorrect.