Since we have no way to detect our presentation (text/emoji), we need to
actually render the glyph that is being requested to double-check that
the glyph matches our supported presentation.
We do this because the browser will render fallback fonts for a glyph if
it can't find one in the named font.
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!
