Animated user dot should pass through batched updates

[1ec5]

After #3589 and #1582 land, the map and/or user dot will animate smoothly in response to user location updates. Sometimes location updates get backed up; Core Location sends these updates in a batch once it catches up. However, -[MGLMapView locationManager:didUpdateLocations:animated:] drops these intermediate locations, animating directly to the latest location.

MGLMapView should take a path through the intermediate locations. This way, if the Core Location backup occurred along a curved road, the user dot won’t appear to take a shortcut. For bonus points, we could add an API to mbgl::Transform for animating along an arbitrary path.

/cc @friedbunny @tobrun

[stale]

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