added wxWindow::IsDoubleBuffered() and improve wxBufferedDC (patch 1565330)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41810 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/window.h

@@ -420,6 +420,9 @@ public:
     // check if mouse is in the window
     bool IsMouseInWindow() const;
 
+    // check if a native double-buffering applies for this window
+    virtual bool IsDoubleBuffered() const;
+
     // synthesize a wxEVT_LEAVE_WINDOW event and set m_mouseInWindow to false
     void GenerateMouseLeave();