added alpha channel support to wxGTK2's wxBitmap

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@28851 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk1/dcmemory.cpp

@@ -71,6 +71,10 @@ void wxMemoryDC::SelectObject( const wxBitmap& bitmap )
             m_window = m_selected.GetBitmap();
         }
 
+#ifdef __WXGTK20__
+        m_selected.PurgeOtherRepresentations(wxBitmap::Pixmap);
+#endif
+
         m_isMemDC = TRUE;
 
         SetUpDC();