Font rendering rewrite

I've rewritten the entire font rendering backend. It now uses texture atlasing to avoid any silly blitting after init. The cost of rendering text now is just creating a vertex buffer, uploading that and draw lots of triangles.

The font renderer in GL is now also done so that multiple font rendering contexts can be active at the same time. Killed off tons of ugly duplicated code in Lakka. Fonts now persist across context reinit as well.

All drivers using fonts, except for vg.c should be updated as well.