expose property transitions to platform SDKs

[incanus]

As in GL JS, we support paint property transitions (i.e animations over time) in core (see here) but not yet in the platform SDKs.

This is an enhancement to runtime styling, but not needed to ship that feature.

/cc @1ec5 @mollymerp @peterqliu

[1ec5]

mbgl does support paint property transitions baked into the style. However, as far as I can tell, the concrete mbgl::style::Layer classes don’t support transitions as part of runtime styling. (They also doesn’t support setting properties per style class: #5948.)

[1ec5]

Also, mbgl does support setting a global paint property duration and delay, via mbgl::Map::setTransitionOptions(). However, in the iOS and macOS SDKs, -[MGLStyle setStyleClasses:transitionDuration:] is currently private; its only caller is -[MGLStyle setStyleClasses:].

[incanus]

I've done some experiments with this using the approach outlined here (under Animating Non-Visual Properties), which leverages CALayer's ability to interpolate various types of values (numbers, colors, etc.) and, within a CABasicAnimation, repeatedly sets them in the runtime styling API through the use of the presentationLayer values.

This isn't high priority right now, but I will get back to this in the near future.

[incanus]

Similarly, @cammace was able to do something a lot like this on Android using ValueAnimator in the demo app. We could probably build out similar APIs on both platforms using the built-in animators of each.

[incanus]

Re: #6779 (comment), I'm going to sidestep anything related to style classes right now due to #5948 (comment).

Instead, I'm working on global/per-map setters & getters that can be used for broad style URL changes as well as runtime styling. I've confirmed that such things work when set globally (so, you could set a global duration of five seconds, then change a runtime styling property and see it animate in over that duration).