Mesh-based text-from-spritesheet renderer

I made a mesh-based text renderer. It can build a "spritesheet" texture of glyphs and then use this to render text as a mesh of quads. There is a small performance improvement for me, but should be more noticable on lower-memory devices.

Also included is some work on building alpha_tested fonts for cleaner edges when text has to be scaled (e.g. in the 3d scene)