Fix retrieving clipping box after changing wxGCDC coordinates (MSW, GTK)

Member data containing clipping box have to be updated not only when the clipping region is explicitly changed by SetClippingRegion()/DestroyClippingRegion() but also when existing wxGraphicsContext is associated with wxGCDC using SetGraphicsContext() or when wxGCDC coordinates are transformed with SetDeviceOrigin(), SetLogicalOrigin(), SetUserScale() or SetLogicalScale().
When any of these functions is called then clipping box data are marked as invalid and retrieved from underlying graphics context using wxGraphicsContext::GetClipBox() at nearest call to GetClippingBox().

See #17646.

include/wx/dcgraph.h

@@ -185,6 +185,8 @@ public:
     virtual void DoSetDeviceClippingRegion(const wxRegion& region) wxOVERRIDE;
     virtual void DoSetClippingRegion(wxCoord x, wxCoord y,
         wxCoord width, wxCoord height) wxOVERRIDE;
+    virtual void DoGetClippingBox(wxCoord *x, wxCoord *y,
+                                  wxCoord *w, wxCoord *h) const wxOVERRIDE;
 
     virtual void DoGetTextExtent(const wxString& string,
         wxCoord *x, wxCoord *y,
@@ -198,6 +200,9 @@ public:
     virtual wxRect MSWApplyGDIPlusTransform(const wxRect& r) const;
 #endif // __WXMSW__
 
+    // update the internal clip box variables
+    void UpdateClipBox();
+
 protected:
     // unused int parameter distinguishes this version, which does not create a
     // wxGraphicsContext, in the expectation that the derived class will do it
@@ -212,6 +217,8 @@ protected:
 
     wxGraphicsContext* m_graphicContext;
 
+    bool m_isClipBoxValid;
+
 private:
     void Init(wxGraphicsContext*);