[racket] fruit flies

Time isn't the only measure.  And in terms of space, I don't think
you're accounting for all of it.

When you make universe scenes bigger, you tend to see GC pressure.
This *suggests* to me that there is not a lot of sharing from one
display to the next; rather, an entire WIDTHxHEIGHT bitmap is being
generated (and thrown away) each time.  Yet I'm pretty certain this is
not how the DOM works.  That means the space difference is much more
than the space for representing the circles.

At least on laptops, increasing the size of the scene seems to induce
more GC, and for large enough images the GC can actually induce a
pause in an animation.  I'm pretty sure this is a qualitative
difference between the two models.

Shriram