impl std::iter::DoubleEndedIterator for palette::gradient::Take

[jansegre]

Would allow doing gradient.take(n).rev() without collecting. My main reasoning for that is basically because for lower values of n (1 or 2), gradient.take(n) is a bit skewed to the first colors of the gradient.

[Ogeon]

Makes sense to me. I see no reason why it wouldn't be double ended.

As for the skewing, I suspect there may have been a mistake when it was implemented and that the last stop is not at the very end of the gradient. That should be fixed.