SpriteFont Refactor + MSDF Fonts

[NoelFB]

I'd like to add support for MSDF fonts using MSDF-Gen and tie it gracefully into SpriteFont.

I think I may remove the background rendering of spritefont characters and require them to be created upfront, as how they are right now is very error prone and hard to maintain. I don't think the cost of having to construct the font upfront is actually very hard, and you could do that process off-thread on your own if you wish.