SVG HSL colours

The new Flex/Lemon parse reads HSL and would be easy to HSLA but currently does nothing with the result, this because the SVG API uses RGB exclusively (makes sense, that used to be the only format supported) so that would need extending.

Then we could then update the convertors to pass HSV to those formats supporting it. Note that the SVG colours may be HSV, but SVG interpolation is still in RGB.

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