Fix session-event queue (multiple writer, single reader)

The reading is done in rt-process thread, but multiple UIs (surfaces,
GUI) can produce events to be queued.

libs/ardour/ardour/session_event.h

@@ -167,6 +167,8 @@ protected:
 	Events           immediate_events;
 	Events::iterator next_event;
 
+	Glib::Threads::Mutex rb_write_lock;
+
 	/* there can only ever be one of each of these */
 
 	SessionEvent *auto_loop_event;

libs/ardour/session_process.cc

@@ -1033,6 +1033,7 @@ Session::queue_event (SessionEvent* ev)
 	} else if (_state_of_the_state & Loading) {
 		merge_event (ev);
 	} else {
+		Glib::Threads::Mutex::Lock lm (rb_write_lock);
 		pending_events.write (&ev, 1);
 	}
 }