AC_WPNav: add get_target_alt method

ArduCopter/ArduCopter.pde

@@ -1967,7 +1967,7 @@ void update_throttle_mode(void)
     case THROTTLE_AUTO:
         // auto pilot altitude controller with target altitude held in next_WP.alt
         if(motors.auto_armed() == true) {
-            get_throttle_althold_with_slew(wp_nav.get_destination_alt(), g.auto_velocity_z_min, g.auto_velocity_z_max);
+            get_throttle_althold_with_slew(wp_nav.get_target_alt(), g.auto_velocity_z_min, g.auto_velocity_z_max);
         }
         break;
 

libraries/AC_WPNav/AC_WPNav.h

@@ -39,6 +39,9 @@ public:
     /// get_loiter_target - get loiter target as position vector (from home in cm)
     Vector3f get_loiter_target() { return _target; }
 
+    /// get_target_alt - get loiter's target altitude
+    float get_target_alt() { return _target.z; }
+
     /// set_loiter_target in cm from home
     void set_loiter_target(const Vector3f& position) { _target = position; }