(svn r10984) -Codechange: unify the way one can get the current speed in the same forwat so we can display it.

src/vehicle.h

@@ -414,6 +414,12 @@ struct Vehicle : PoolItem<Vehicle, VehicleID, &_Vehicle_pool> {
 	 */
 	virtual int GetImage(Direction direction) const { return 0; }
 
+	/**
+	 * Gets the speed in mph that can be sent into SetDParam for string processing.
+	 * @return the vehicle's speed
+	 */
+	virtual int GetDisplaySpeed() const { return 0; }
+
 	/**
 	 * Calls the tick handler of the vehicle
 	 */