Allow to retrieve the wxWindow associated with a wxBufferedPaintDC

When a wxBufferedPaintDC is created, the base classes does not initialize the
m_window variable with the used wxWindow. Only the associated m_paintdc
initializes this variable. Add a protected function that allows to set the
wxWindow of a wxDC so GetWindow() will return the window.

This fixes the font size of custom attributes in wxDataViewCtrl when the DPI
changes.

include/wx/dc.h

@@ -269,6 +269,8 @@ public:
 
     wxWindow* GetWindow() const { return m_window; }
 
+    void SetWindow(wxWindow* w) { m_window = w; }
+
     virtual bool IsOk() const { return m_ok; }
 
     // query capabilities
@@ -1364,6 +1366,9 @@ protected:
 
     wxDCImpl * const m_pimpl;
 
+    void SetWindow(wxWindow* w)
+        { return m_pimpl->SetWindow(w); }
+
 private:
     wxDECLARE_ABSTRACT_CLASS(wxDC);
     wxDECLARE_NO_COPY_CLASS(wxDC);