MGLMapView now maps “annotation tags” (Cocoa-speak for numeric annotation IDs) to context objects that pair annotations with their identifiers. Previously, MGLMapView mapped the annotation objects themselves to context objects that contained annotation IDs, but this caused most operations involving annotations to be rather expensive: since mbgl tracks annotations by ID, most of the time you know the ID and need either the annotation or some piece of metadata about it.
Along for the ride, geographic methods and support for alignment rect insets on annotation images have been ported from #3135 for OS X. Hit testing now works much like in #3135, with the exception that UIKit doesn’t support hit testing based on pixel opacity. -convertRect:toLatLngBoundsFromView: returns a world-spanning bounds when the world is wrapping.
Fixes#3159.
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