Fix animations with GLViews (#931)

* null guard _displayLink (bz42755)

* change NSRunLoop mode (fix bz41413)

Xamarin.Forms.Platform.iOS/Renderers/OpenGLViewRenderer.cs

@@ -76,12 +76,12 @@ namespace Xamarin.Forms.Platform.iOS
 					control.Display();
 				if (control == null || model == null || !model.HasRenderLoop)
 				{
-					_displayLink.Invalidate();
-					_displayLink.Dispose();
+					_displayLink?.Invalidate();
+					_displayLink?.Dispose();
 					_displayLink = null;
 				}
 			});
-			_displayLink.AddToRunLoop(NSRunLoop.Current, NSRunLoop.NSDefaultRunLoopMode);
+			_displayLink.AddToRunLoop(NSRunLoop.Current, NSRunLoop.NSRunLoopCommonModes);
 		}
 
 		class Delegate : GLKViewDelegate