added Interpolation support to ColourPoint and corresponding method

[benkuper]

This is a proposition to add interpolation support to ColourGradient. It adds an enum ColourGradient::Interpolation (only LINEAR and and NONE for now, LINEAR being the current interpolation method, there could be SMOOTH (Ease in-out))

The only thing I can't work out is the rendering of fillGradient, because it seems to rely on lowlevel graphics methods that I don't know at all.

Hope it helps !