adding magnification API into the wxWindow classes for best retina support

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74247 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/window.h

@@ -521,6 +521,11 @@ public:
         return wxSize( wxMax( client.x, best.x ), wxMax( client.y, best.y ) );
     }
 
+    // returns the magnification of the backing store of this window
+    // eg 2.0 for a window on a retina screen
+    virtual double GetMagnificationFactor() const
+    { return 1.0; }
+    
     // return the size of the left/right and top/bottom borders in x and y
     // components of the result respectively
     virtual wxSize GetWindowBorderSize() const;