don't modify all wxBitmaps sharing the same wxBitmapRefData when SetMask() is called

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@28434 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/bitmap.h

@@ -179,6 +179,7 @@ protected:
     void Init();
 
     virtual wxGDIImageRefData *CreateData() const;
+    virtual wxObjectRefData *CloneRefData(const wxObjectRefData *data) const;
 
     // creates the bitmap from XPM data, supposed to be called from ctor
     bool CreateFromXpm(const char **bits);