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!
This is more oriented around glyph indexes and also introduces an
important concept in the FontIndex which can be cached ahead of time so
that we can eventually break down text into runs for text shaping.
