add framerate counter

[dyoo]

Emmanuel writes:

Load times were slow, but since it only happens once it's not so bad. The big speed hit was running World animations - mind you, these were on old P3s with like 256mb of RAM using FF3.5. Even the prototype game, which is just circles and rectangles moving along one axis, seemed pretty slow.

I suggested that Chris try installing Chrome on the machines, since Chrome is both much faster and has VASTLY better memory use.

It might be handy for perf reports to write the framerate to window.status - incrementing a counter every time the canvas is drawn and then dividing be something reasonable like 2-3 seconds.