Quit the Mac app after the window is closed

samples/Skia.OSX.Demo/AppDelegate.cs

@@ -19,6 +19,11 @@ namespace Skia.OSX.Demo
 		{
 			// Insert code here to tear down your application
 		}
+
+		public override bool ApplicationShouldTerminateAfterLastWindowClosed (NSApplication sender)
+		{
+			return true;
+		}
 	}
 }