Add void *layer to Demos AppleWindow

layer is required for "VK_EXT_metal_surface"

util/test/demos/apple/apple_window.cpp

@@ -153,7 +153,9 @@ AppleWindow::~AppleWindow()
 AppleWindow::AppleWindow(int width, int height, const char *title) : GraphicsWindow(title)
 {
   pAppDelegate->CreateWindow(width, height, title);
-  view = pAppDelegate->GetContentView();
+  NS::View *nsView = pAppDelegate->GetContentView();
+  view = nsView;
+  layer = nsView->layer();
 }
 
 bool AppleWindow::Init()

util/test/demos/apple/apple_window.h

@@ -40,4 +40,5 @@ struct AppleWindow : GraphicsWindow
 
   static MyAppDelegate *pAppDelegate;
   void *view;
+  void *layer;
 };