image-ssim call is borked

[patrickhulce]

I have a PR in progress to fix, but basically the issue is https://github.com/pmdartus/speedline/blob/master/src/speed-index.js#L50 interacting with https://github.com/darosh/image-ssim-js/blob/master/index.ts#L155. The image being read to determine SSIM is not the actual image because it's being interpreted in pixel-sets of 3 when the encoding is pixel-sets of 4 with the 4th being the alpha channel which is always 1.