MGLStyleFunction needs a colorSpaceModel property

[1ec5]

The iOS and macOS SDKs need to support the new color interpolation option being added in #6442 #8720 via a new colorSpaceModel property on MGLStyleFunction. No need to extend the initializers, since we can assume a default color space model of RGB.

Ideally, the property would be of type CGColorSpaceModel or NSColorSpaceModel. However, we’d have to provide our own HCL color space model and document the fact that some of the common color space models provided by Core Graphics or AppKit, like grayscale, are unavailable. So it might be easier, if less cool, to define our own enumeration.

/cc @friedbunny @tmcw @zugaldia

[1ec5]

I think we probably will have to provide our own enumeration for this type, because CGColorSpaceModel is new as of iOS 9 and macOS 10.11, and NSColorSpaceModel is new as of macOS 10.10. But we can be consistent with naming at least.

[jfirebaugh]

We removed functions. If we bring back color spaces, it'll be as an expression.

[1ec5]

#8720 has been repurposed to track the addition of an expression operator. There would still need to be some platform-specific work, so this issue could be reopened to track that when the time comes.