[racket] Graphics Issues in Ubuntu 11.10

It seems that `racket/gui' is trying to freeze and thaw window updates
before the canvas is mapped. The relevant Gdk window changes when the
canvas is mapped, which means that Gdk and `racket/gui' end up with
different opinions on the freeze/thaw state of the canvas.

I've pushed a repair to fix freeze/thaw requests based on the canvas's
mapped state, which hopefully fixes the problem that you're seeing.