Plane: set initial alt target on takeoff completion

this prevents a sudden motor spike at the start of the transition

ArduPlane/quadplane.cpp

@@ -1754,6 +1754,7 @@ bool QuadPlane::verify_vtol_takeoff(const AP_Mission::Mission_Command &cmd)
     }
     transition_state = TRANSITION_AIRSPEED_WAIT;
     plane.TECS_controller.set_pitch_max_limit(transition_pitch_max);
+    pos_control->set_alt_target(inertial_nav.get_altitude());
 
     plane.complete_auto_takeoff();