Need to premultiply alpha. Also fixed bug in slow path.
Wire up JRebel.

Man Eclim plus JRebel is really awesome. I make changes to code in emacs, and I
see them in my running program. There are no intermediate steps.
Added a canvas stress test (from Daniel Gerson).

It basically just does some insanely inefficient shit with a canvas image, and
if the backend bogs down as a result, we know that our canvas image data
uploading is using the slow path.