Specify color space when generating gradient

[Thomasch]

When I changed to use P3 wide colors in our app, I noticed that the gradients didn't use that color space.

I looked through the code and saw that MKGradientGenerator uses CGColorSpaceCreateDeviceRGB. I changed that to CGColorSpace(name: CGColorSpace.displayP3) and the gradient was displaying in P3.

Maybe you can add a parameter (with default value) or get the color space from the start/end color?

Thanks!

[maxkonovalov]

@Thomasch wide colors are not really supported by the lib at the moment and all color components are clipped to [0.0, 1.0] range. So I think it makes no sense to add P3 color space support just yet.