font blank without ttf-dejavu

[tkkcc]

cargo run in examples/widget-gallery gives blank font, no warn or error. environment is archlinux, closely related to https://github.com/lapce/lapce/issues/607

1. tried rename ~/.config/fontconfig, no help
2. after install ttf-dejavu, the font shows. i dont use this font, and have no font rendering issue with alacritty(default config, so it use monospace) and gtk or qt or webview based guis(they all respect fontconfig).
3. this library seems not respect fontconfig

my fontconfig:

<fontconfig>
    <match>
        <test qual='all' name='family' compare='not_eq'><string>ZhunYuan</string></test>
        <edit name='family' mode="assign_replace">
         <string>Consolas Nerd Font</string>
          <string>JetBrainsMono Nerd Font</string>
         <string>Microsoft YaHei</string>
        </edit>
    </match>
</fontconfig>