gerbview: Ensure selection lives on top

Just picking the next layer will be overwritten when we have multiple
objects on the same layer in gerbview.  Move the selection to the top to
ensure it is out top-most layer when drawing.

Fixes: lp:1833868
* https://bugs.launchpad.net/kicad/+bug/1833868

gerbview/tools/gerbview_selection_tool.cpp

@@ -525,7 +525,7 @@ void GERBVIEW_SELECTION_TOOL::select( EDA_ITEM* aItem )
         return;
 
     m_selection.Add( aItem );
-    getView()->Add( &m_selection );
+    getView()->Add( &m_selection, std::numeric_limits<int>::max() );
     selectVisually( aItem );
 }