Additional image diff mode: "blend-mask"

This came from a discussion with @lbennett involving this article: http://jeffkreeftmeijer.com/2011/comparing-images-and-creating-image-diffs/

Currently we have 3 image diff modes we can use: onion skin, side-by-side and swipe.

Proposal

To add an additional blend mode to this array of diffing modes for images.

Why?

Blend mode will allow to spot smaller differences like, color differences and really small areas.

Result would be similar to:

with UI like:

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@lbennett was looking into this tool for CI purposes already..

Code examples and documentation can be found in the article

cc: @victorwu @awhildy @jschatz1