Extremely slow tests when comparing screenshots of different sizes

[JoseAlcerreca]

Consider checking the size and fail immediately. Since the comparison is made on device, a slow emulator can take a long time (I'm getting 37s per test).

We would lose the 3-way diff I assume.

[rharter]

The diff performance is dependent on the number of pixels that need to be compared, which ends up being a lot when the images are completely different. This is a mix of comparing things that we already know aren't the right size, but also of the default configuration of the image comparator, which uses a large shift window that multiplies the number of compares that are needed. I'll be fixing this in two parts in the coming PRs.