Font code Improvement

[Zefz]

Continue work on #218:

Rewrite how TrueType fonts are handled; the code now uses atlases and vertex buffers instead of a texture for each string of text.

TODO:

Finish adding support for the new layoutText API in GUI
Relayout text as needed
Remove caches for sizeText and drawText
TODO, eventually:

Move the UTF-8 to codepoints code and expand it to support all UTF-8-encodable codepoints (SDL_ttf only supports 16-bit codepoints)
Split the Font class into a base Font class and TTFont and BitmapFont subclasses