Call base class version from overridden DestroyClippingRegion()

No real changes, just call wxDCImpl::DestroyClippingRegion() from
the overridden versions in the derived classes instead of calling
ResetClipping(): this makes the code more clear as it follows the usual
pattern of the derived class doing something first and then forwarding
to the base class.

Also, as ResetClipping() is not really useful, add a comment documenting
that it shouldn't be used in the new code.

src/common/dcgraph.cpp

@@ -376,7 +376,7 @@ void wxGCDCImpl::DestroyClippingRegion()
     m_graphicContext->SetPen( m_pen );
     m_graphicContext->SetBrush( m_brush );
 
-    ResetClipping();
+    wxDCImpl::DestroyClippingRegion();
     m_isClipBoxValid = false;
 }