Add wxDCImpl::MSWApplyGDIPlusTransform() to formalize wxRendererMSW hack.

Replace the dynamic_cast<> used in wxMSW wxRenderer implementation code with a
virtual function call.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68180 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dcgraph.h

@@ -193,6 +193,10 @@ public:
 
     virtual bool DoGetPartialTextExtents(const wxString& text, wxArrayInt& widths) const;
 
+#ifdef __WXMSW__
+    virtual wxRect MSWApplyGDIPlusTransform(const wxRect& r) const;
+#endif // __WXMSW__
+
 protected:
     // scaling variables
     bool m_logicalFunctionSupported;