Pixi integration

[mattdesl]

Some users have requested a pure Pixi implementation of:

• simple bitmap rendering using PIXI.Texture etc
• signed distance field rendering for smooth scaling

This could be accomplished with a pixi-bmfont-text module like the ThreeJS one. It's best to wait for Pixi v3 as shaders and static buffers are changing there.

If there is a lot of shared code it could be split into a module, e.g. bmfont-text-vertex-data or something.