Improve diff algorithm for grayscale changes

With the starfield/night, improving grayscale diffing is more important, so I took a shot at it.

I ended up with two different scales/algorithms, which are both included with comments - the one I left enabled is an inverse square root, which should emphasize small changes at the cost of lumping together bigger changes.  I think the end result works nicely, at least.

Also removed some debug from downloadFrames I inadvertently left in.