Fontconfig in particular appears unsafe to initialize multiple times.
Font discovery is a singleton object in an application and only ever
accessed from the main thread so we can work around this by only
initializing and caching the font discovery mechanism exactly once on
the app singleton.
This implements font discovery so the `--font-family` flag works for macOS. Fonts are looked up using the Core Text API so any installed font on the Mac system can be used.
We still use FreeType for rendering, and CoreText doesn't _quite_ give us all the information we need to build the exact face in FreeType. So a TODO after this is to now implement glyph _rendering_ using Core Text and Core Graphics. Until then, a couple fonts don't quite work (i.e. Monaco, a big one!) but many do!
