Provide SSIM image (structural similarity index measure) to user

[garywill]

Is your feature request related to a problem? Please describe.
Users are always uncertain about how big the difference is between the image before and after compression, how much the photo quality has decreased, and whether it has affected the preservation of key information in the photo.

Describe the solution you'd like
Compute difference by SSIM algorithm (or something else) and provide SSIM image to user.

Describe alternatives you've considered
Butteraugli heatmap is also an alternative. But Butteraugli seems slow.

Additional context
The example images are from https://stackoverflow.com/questions/56183201/detect-and-visualize-differences-between-two-images-with-opencv-python

SSIM https://en.wikipedia.org/wiki/Structural_similarity_index_measure

Butteraugli estimates the psychovisual difference between two images https://github.com/google/butteraugli

Guetzli implements butteraugli. Guetzli is a command-line app. open-source . Windows, macOS, and Linux versions of Guetzli are directly available https://github.com/google/guetzli

[Lymphatus]

That's an interesting one. Since the core library of Caesium is in Rust I was using DSSIM in my internal tests, but it only outputs a value and not the diff image which might not be ideal.
I will experiment a bit with a couple of libraries. I would not use butteraugli though, because it's very slow and the project is archived.