Implement Colour DIfference Formula

[gennyble]

Found Wikipedia: Color Difference while looking at GitHub: enbyss/colour-exercise-rs. I did not find this wikipedia page before and I am quite annoyed about that, but it's fine.

[gennyble]

readmean implemented in 2f3b95d.

not to be surprised that a real, actual colour difference algorithm worked better than "hey, lol, what if I sum the differences" but wow the results are really good.

edit:
the results are good! but that was probably just because we chose a good tolerance as we didn't have one for that algorithm. We still need a way to guess at a good number. see: https://github.com/gennyble/colorsquash/blob/2f3b95dbb4124fbb3c51b5ecec9fdbe560a9a3ba/src/lib.rs#L131-L135

[gennyble]

redmean and rgb_difference (shown below) both return values between 0 and 765. When I adjust the tolerance for rgb_difference to 0.04 * 768.0 I get similar results to redmean.

We could take a tolerance 0-100 to mean "percent different" probably.

https://github.com/gennyble/colorsquash/blob/2f3b95dbb4124fbb3c51b5ecec9fdbe560a9a3ba/src/lib.rs#L239-L242