Font refactoring, font fallbacks

[jlnr]

This branch is the continuation of #429:

• Refactor new text parsing code to use normalized UCS-4 instead of graphemes, since our TTF rendering code cannot handle complex graphemes anyway (bummer).
• Refactor new TTF code to add font fallback logic, otherwise CJK text can't be rendered using Gosu.default_font_name :/
• Fix markup (I think bold is broken right now?)
• Fix dark pixels around text on translucent background
• Fix missing text/weird offsets in text-markup-1.png
• Fix dark spots where letters overlap
• Fix tests (even the ones I broke on master) (I think I should do a separate, focused test PR – right now it's OK for me if the tests only work on macOS)