Incorporate test images into tests

[jywarren]

Closed #1 as it is a bit out of date now, but I still would like tests which incorporate a "before" and "after" image, as attempted in this code:

https://github.com/jywarren/image-sequencer/blob/3d43eceff9a51ed2f85c3ce42277017c7890e435/test/image-sequencer.js#L31-L59

At simplest, we could perform an operation, then reverse it -- like, for example, inverting and inverting again -- and then confirm that the image is the same after the test, perhaps using:

• https://www.npmjs.com/package/blink-diff
• smaller and faster: https://www.npmjs.com/package/@schornio/pixelmatch

Or even as an initial step, do an operation on an image and expect() that it's not the same -- just confirm that /something/ happened. Then move on to more complex tests of actual test images.

If this is in another PR, please link it here, thanks!

[ccpandhare]

I was onto it. Got stuck at the PixelManipulation thing yet again.

[ccpandhare]

Now that we're done with that, I think this can be worked upon!