Add Font Module

[billyquith]

Not all renderers have good support for fonts. Basic support has been added using the STB font code. It would be nice to use something like FontStash to provide a more complete solution.

This can be shared between renderers if the client renderer supports:

• Render glyph using DrawTextureRect().
• Render to texture, so that newly cached glyphs can be cached in a renderer specific way.