Fix #6676: Prevent helicopters from stopping in midair during some kinds of landing

This adds a new flag that gets stored in the savegame, but it should still be compatible both ways, hence no save version bump.

src/aircraft.h

@@ -45,6 +45,8 @@ enum AirVehicleFlags {
 	 * landscape at a fixed altitude. This only has effect when there are more than 15 height levels. */
 	VAF_IN_MAX_HEIGHT_CORRECTION = 1, ///< The vehicle is currently lowering its altitude because it hit the upper bound.
 	VAF_IN_MIN_HEIGHT_CORRECTION = 2, ///< The vehicle is currently raising its altitude because it hit the lower bound.
+
+	VAF_HELI_DIRECT_DESCENT      = 3, ///< The helicopter is descending directly at its destination (helipad or in front of hangar)
 };
 
 static const int ROTOR_Z_OFFSET         = 5;    ///< Z Offset between helicopter- and rotorsprite.