[ios] Disable annotation view implicit animation

Like #3683, this
change prevents Core Animation from implicitly animating changes to the
annotation's bounds. This is done to reduce lag during map interactions
like pan and drag.

platform/ios/src/MGLAnnotationView.m

@@ -9,4 +9,14 @@ @interface MGLAnnotationView ()
 
 @implementation MGLAnnotationView
 
+- (id<CAAction>)actionForLayer:(CALayer *)layer forKey:(NSString *)event
+{
+    // Allow mbgl to drive animation of this view’s bounds.
+    if ([event isEqualToString:@"bounds"])
+    {
+        return [NSNull null];
+    }
+    return [super actionForLayer:layer forKey:event];
+}
+
 @end