Switch to using DoGetClippingRect() instead of DoGetClippingBox()

The new method returns a boolean flag which indicates whether there is
any clipping region or not and so is preferable to using the old one and
checking its return value to determine this, which can't be done
reliably.

include/wx/dc.h

@@ -453,11 +453,15 @@ public:
     // up to date.
     virtual bool DoGetClippingRect(wxRect& rect) const;
 
+#if WXWIN_COMPATIBILITY_3_0
     // This method is kept for backwards compatibility but shouldn't be used
     // nor overridden in the new code, implement DoGetClippingRect() above
     // instead.
-    virtual void DoGetClippingBox(wxCoord *x, wxCoord *y,
-                                  wxCoord *w, wxCoord *h) const;
+    wxDEPRECATED_BUT_USED_INTERNALLY(
+        virtual void DoGetClippingBox(wxCoord *x, wxCoord *y,
+                                      wxCoord *w, wxCoord *h) const
+    );
+#endif // WXWIN_COMPATIBILITY_3_0
 
     virtual void DestroyClippingRegion() { ResetClipping(); }