Fix tempo-marker/curve z-axis stacking

Previously it was possible that the tempo-curve display was rendered
above a tempo-marker (e.g. create a marker in the between of existing
markers). With the tempo-curve in front, markers on the right
became inaccessible.

gtk2_ardour/editor_tempodisplay.cc

@@ -427,6 +427,7 @@ Editor::update_tempo_curves (double min_tempo, double max_tempo, samplecnt_t sr)
 		++tmp;
 
 		TempoCurve& curve (tm->curve());
+		curve.the_item().lower_to_bottom ();
 
 		curve.set_max_tempo (max_tempo);
 		curve.set_min_tempo (min_tempo);