fix behavior when add new meter events

We need to use a BBT reference based on existing BBT markers, not zero.

gtk2_ardour/editor_tempodisplay.cc

@@ -472,9 +472,12 @@ Editor::mouse_add_new_meter_event (timepos_t pos)
 
 	Temporal::BBT_Time r;
 	meter_dialog.get_bbt_time (r);
-	Temporal::BBT_Argument requested (superclock_t (0), r);
 
 	TempoMapChange tmc (*this, _("add time signature"));
+	superclock_t sc (tmc.map().previous_bbt_reference_at_superclock (samples_to_superclock (pos.samples(), TEMPORAL_SAMPLE_RATE)));
+
+	Temporal::BBT_Argument requested (sc, r);
+
 	pos = timepos_t (tmc.map().quarters_at (requested));
 	tmc.map().set_meter (Meter (bpb, note_type), pos);
 }