Font Fallbacks

Add a way to use multiple `Rasterizer` objects with a font. Example use cases are providing a fallback for glyphs that don't exist in the main font, or for supporting icons by supplementing a regular font with a BMFont that contains icons in an atlas. - Unclear if this should conceptually be "a fallback Rasterizer for the Font", a list of fallback rasterizers, or just "Font has a list of Rasterizers that are tried in order, instead of a single one". - The SDF-ness of the rasterizers probably need to match. - Can you mix BMFont and TTF fonts? Seems okay? - The glyphs from all the rasterizers still go in a single atlas, right? - LÖVE prior art: `Font:setFallbacks(...Font)`. Why does it take Font instead of Rasterizer though? - Do you have to set the fallbacks at Font creation time, or can you change them at runtime like LÖVE? Changing them at runtime seems more complicated.