Call default_substitute after config_substitute

[wezm]

The Fontconfig documentation doesn't specify the order, instead just saying:

This function should be called only after FcConfigSubstitute and FcDefaultSubstitute have been called for pattern; otherwise the results will not be correct.

However, a code search on GitHub shows that FcDefaultSubstitute is typically called after FcConfigSubstitute, as do the tools in Fontconfig itself.

Fixes #46