Added Structual SIMilarity calculation in a JS file. Used for checking

SSIM between contiguous frames in a near-frozen frames subtest of camtest.js.

PTAL :)
Should also use curly bracers " {}" even when there is only one line after an if statement.

FYI: Dev guideline in progress [here](https://docs.google.com/a/google.com/document/d/1tn1t6LW2ffzGuYTK3366w1fhTkkzsSvHsBnOHoDfRzY/edit#heading=h.yamj1q74ij6c)