Overdraw visualizer

• Only available in latest dev build

Toggle overdraw rendering is be extremely useful for performance tuning. Especially since the platforms where Flambe games generally run are very fillrate-bound. Just press ctrl-o to enable the visualizer. To disable it, press ctrl-o again.

See how overdraw tuning helped in a (non-flambe) game: http://joostdevblog.blogspot.nl/2013/02/optimising-special-effects-in.html