Fixed missing Mode value in CocoaContext.

Crashed E.g. GameWindow States example.

Source/OpenTK/Platform/MacOS/CocoaContext.cs

@@ -149,6 +149,7 @@ namespace OpenTK
 
             // Finalize
             Handle = new ContextHandle(context);
+            Mode = mode;
             Update(cocoaWindow);
             MakeCurrent(cocoaWindow);
         }