Canvas: clear elements in shutdown()

- without this change, elements aren't un-ref-ed and hence destroyed
until the actual dtor runs, which is too late (other systems may be
destroyed already)

simgear/props/PropertyBasedMgr.cxx

@@ -37,6 +37,7 @@ namespace simgear
   void PropertyBasedMgr::shutdown()
   {
     _props->removeChangeListener(this);
+    _elements.clear();
   }
 
   //----------------------------------------------------------------------------