Call FcConfigSubstitute and FcDefaultSubstitute before FcFontMatch
Call FcConfigSubstitute and FcDefaultSubstitute before FcFontMatch

According to the [FcFontMatch documentation](https://www.freedesktop.org/software/fontconfig/fontconfig-devel/fcfontmatch.html), fontconfig can return
wrong results if these functions have not been called before.

Note: When running the regression tests on my local machine with
this change, I get some diffs for `bug-264b.sil` and `vertical.sil`.
However, when building SILE from git head without any changes,
I get the exact same errors.

alerque/sile,neofob/sile,simoncozens/sile,neofob/sile,neofob/sile,simoncozens/sile,simoncozens/sile,simoncozens/sile,neofob/sile,alerque/sile,alerque/sile,alerque/sile