Font export

[kpreid]

(This issue was split off from #424.)

Implement exporting fonts to .ttf. This serves the following purposes:

• Allow the same fonts to be used in renderers we don’t control:
  • Web version’s text overlay
  • Any other "normal web page" UI we may build
  • One of the ways I’m considering handling text input is via overlaying native text box widgets, in which case we’ll want to supply a font to it.
• Reduce NIH:
  • Allow empirical comparison of how our invented font system works to mainstream fonts.
  • Become more familiar with how font formats work.
  • A natural companion to importing fonts (though TTF in particular is not a great choice for acquiring true pixel fonts, since it’s too flexible).

This does not serve the following purposes:

• Actually make a game