Child GLKView should be bottommost child

Otherwise, any subviews added to an MGLMapView in a XIB or storyboard are completely obscured by the GLKView.

platform/ios/MGLMapView.mm

@@ -212,7 +212,7 @@ mbgl::DefaultFileSource *mbglFileSource = nullptr;
     }
     _glView.delegate = self;
     [_glView bindDrawable];
-    [self addSubview:_glView];
+    [self insertSubview:_glView atIndex:0];
 
     _glView.contentMode = UIViewContentModeCenter;
     [self setBackgroundColor:[UIColor clearColor]];