[ios] Introduce MGLAnnotationView and support for view annotations

Add an UIView subclass that can be used as the base class for all
client provided UIViews for annotations.

Teach MGLMapView to be able to display annotation views over the map if provided by the client delegate. For now, if the delegate provides a UIView then it will be used. If not, the map view will fall back to the old strategy of using GL annotations with an image provided by the delegate or a default image if not.
The map keeps a reuse queue and will store annotation views that are panned
offscreen in the queue if the application developer supplied a reuse queue identifer. The views in the queue are reused when more annotation
views are required. This view reuse provides a performance gain when many
annotations are shown and most of them are offscreen.

iosapp now implements the new delegate method to supply
a native view.

Add a playground to the workspace to facilitate experimentation
with new features. A playground is capable of importing frameworks if it
exists in the same workspace that builds the imported framework. The
initial playground demonstrates annotation views.

This also fixes a crash due to nullptr in annotations array If the `annotations` method is called while the user dot's callout view was showing, the userdot annotation is represented as null in the annotation context map. This
caused a crash when the null pointer was attempted to be converted
into an NSArray via C array. This protects against this bug by filtering out such a null annotation.

1 files changed, 1 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index 91df4776b7..d887dd3f90 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,4 @@ xcuserdata
 /node_modules
 /platform/ios/benchmark/assets/glyphs/DIN*
 /platform/ios/benchmark/assets/tiles/mapbox.mapbox-terrain-v2,mapbox.mapbox-streets-v6
+**/token