Custom callout: stay open during panning (and some refactoring)

[friedbunny]

Implements MGLCalloutView’s anchoredToAnnotation and dismissesAutomatically from mapbox/mapbox-gl-native#7646 in our custom callout view example.

Because of mapbox/mapbox-gl-native#9228, this also overrides the center property of the custom callout view, otherwise its position will shift undesirably when the user first interacts with the map.

Also:

• Selects the annotation in viewDidLoad so that the callout appears automatically.
• Moves the annotation property checks to mapView:annotationCanShowCallout:, which seems like a more natural fit than mapView:calloutViewForAnnotation:.
• Fixes a really annoying intermittent hairline gap between the arrow and label of the callout view. 😑
• Re-adds lazy vars, because that Swift bug has been fixed.
• Updates the project for CocoaPods 1.3.1.

/cc @jmkiley @frederoni @captainbarbosa

[friedbunny]

👋 @jmkiley @frederoni

[jmkiley]

LGTM 🙇‍♀️