Per-attribute transition properties on MGLStyleLayer

[1ec5]

MGLStyleLayer’s concrete subclasses should allow the developer to set transitions that affect individual layout or paint attributes.

We could pair every layout or paint attribute with a transitionFor… property. Or we could fight property proliferation by adding -transitionForKey: and -setTransition:forKey: methods to MGLStyleLayer itself (along the lines of -[CALayer addAnimation:forKey:]). (The downside would be pushing validation of the key until runtime.)

Either way, we should add an MGLTransition class or struct to encapsulate the details of a transition. We’ll want to refine the MGLStyle methods added in #7711 to make use of MGLTransition instead of working with durations and delays directly.

/ref #7863
/cc @incanus @boundsj @jfirebaugh @tobrun

[1ec5]

mbgl::Layer subclasses expose properties for setting transitions on individual layout and paint properties (example). However, there don’t appear to be any getters. We should add transition getters too.

[boundsj]

This was added in #8225 that has landed on the release-ios-v3.5.0-android-v5.0.0 release branch and will be in the next iOS 3.5.0 beta 3 release.