graph-fonts: rewrite, use atlas to draw OSD, severely reduce CPU usage

Severely's an understatement. There's practically no CPU used now, fell down from 40% just idling without physics to less than 2% peak. Turns out uploading a new texture every char is beyond inefficient. There's a small problem with alignment right now but that's hardly visible on a proper monitor... probably. Surely an off-by-one error somewhere. Just need to finish threading BH, making demos and ship v0.2.