Add Support for HSL Gradient Interpolation

[stellarpower]

I'm a new Inkscape user, so I may be wrong, but based on some time searching, it seems that Inkscape doesn't support gradients interpolated using HSL. That is, given two stops in RGB, it will generate the gradient values inbetween by making a gradual progression over the individual R, G and B channel values from the left stop to the right stop. However, it can't generate the values by making a progression over H, S and L values. As far as I can see, this makes 'rainbow' gradients or gradients that change smoothly in hue impossible without using multile stops as an alternative. Adding support for HSL gradients would make this possible, and also facilitate progressions over saturation and luminosity.

Following GIMP's offerings, it seems sensible to suggest that gradients have three modes: RGB, HSL(clockwise) and HSL(anti-clockwise), or two modes and a boolean option for HSL over the direction of variation. With the HSL modes, it would also be a good idea to have a boolean option specifying whether, when two stops have the same hue, the hue should vary by 360 degrees over the gradient (as in GIMP) or wherther the hue should remain constant, allowing variation in only the saturation and luminosity channels. HSV support could be added too in a similar way to HSL.

I remember reading somewhere that Inkscape (or SVG - not sure which) only really operates on RGB values and so understand if this would not be possible to implement.

Thank you for your time!

(Inkscape 0.48.4 r9939 (Jul 30 2013), Linux Mint Petra AMD64)