Add tab navigation for snapshot diff sprite

Now that we save the snapshot diff image as a sprite consisting of three
images, we need to make the rendering a bit smarter. By adding tabs, and
only showing a third of the image at once, we allow a more focused
review. Most of the time, the "Diff" image is enough to make a good
decision about whether or not the diff should be accepted. This is also
the part of the image that we default to.
Add "x" keyboard shortcut to switch diff tab

This makes it simple to flip between the before, diff, and after image
for a snapshot.