Julia SSIM trap

SSIM is a metric used to compare image similarity.

While original SSIM is computed against the luma channel (i.e. in a gray-scale), the Julia language computes a weighted combination of per-channel SSIM's.

This sample is a white image and a gray image that are reported equal by Julia SSIM. It also traps all the suggestions for color-weighted SSIM given here: https://dsp.stackexchange.com/questions/75187/how-to-apply-the-ssim-measure-on-rgb-images