[racket] Facing some flickering problems

Matt, what's missing from Universe to avoid your kind of flickering isn't double buffering. It is a handler that allows you to send updates to an established scene so that you don't have to draw the whole scene over into the canvas. 

(Plus a change to the timing guarantees. I made the mistake of measuring the clock ticks from tick to tick. As Matthew (Flatt) pointed out in January, I should start the next tick at the end of the draw action of the preceding tick. That will simplify your programming model and my internal organization.)