A new colour engine for Auto and Detailed

Auto and Detailed now pick the image's own colours, up to 12 and 16, and trace each one as its own layer. A red and a blue of the same brightness could come back as one colour. Now they stay apart, and flat artwork gets about one layer per colour, plus the background.

Clean / Flat still splits the image into four bands of brightness, so it can still merge such a pair. And a very small patch of a strong colour can still come back as a nearby colour.