extend TempoMapChange API to allow for replacing, not editing, the map

gtk2_ardour/tempo_map_change.cc

@@ -63,3 +63,12 @@ TempoMapChange::abort ()
 		begun = false;
 	}
 }
+
+void
+TempoMapChange::use_new_map (Temporal::TempoMap::WritableSharedPtr new_map)
+{
+	/* existing write copy goes out of scope, and is deleted, We continue
+	 * on with the new map, and will RCU-update it in our destructor.
+	 */
+	writable_map = new_map;
+}

gtk2_ardour/tempo_map_change.h

@@ -41,6 +41,7 @@ class TempoMapChange {
 
 	void begin ();
 	void abort ();
+	void use_new_map (Temporal::TempoMap::WritableSharedPtr);
 
 	Temporal::TempoMap& map() const { return *writable_map.get(); }