Feature Request: Permute as a math evaluation function

[Reptorian1125]

Inspired by this thread - https://discuss.pixls.us/t/sort-images-according-to-n-sized-channels/40480/5 .

I think it would be a good idea to have permute_xyzc function within the math evaluator. This would allow to treat cropped areas as if they were permute of a image. There can also be inverse_permute_xyzc to make it easy to draw the values back though it's not hard to find the inverse permute xyzc, and I have a custom G'MIC command just to do that.