This patch adds a new similarity metric that will allow us to determine when content changes occur in live site tests. It also enabled to recorded sites so we can get a comparison of the quality of the recording (and difference) between it and the live site. There are 2D and 3D variants of this score which capture different things. The 2D score only looks at the final frame, so it gives a measure of how consistent/similar the end state is for the test. The 3D variant is more comprehensive and captures how the page was rendered.
Differential Revision: https://phabricator.services.mozilla.com/D73450
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