Further implementation of native overlay on Cocoa

Should work for Caret,  still has a problem with the transformation matrix otherwise, too many things have changed there for me to find a quick solutions.

include/wx/osx/cocoa/private/overlay.h

@@ -38,7 +38,7 @@ public:
     void Clear( wxDC* dc);
 
 private:
-    void CreateOverlayWindow();
+    void CreateOverlayWindow( wxDC* dc );
 
     WXWindow m_overlayWindow;
     WXWindow m_overlayParentWindow;