Plane: adjust throttle mix for auto landing

use mix-max during landing approach and initial descent, use min in
land final. As discussed with Leonard

ArduPlane/quadplane.cpp

@@ -3608,7 +3608,17 @@ void QuadPlane::update_throttle_mix(void)
         // check for requested descent
         bool descent_not_demanded = pos_control->get_vel_desired_cms().z >= 0.0f;
 
-        if (large_angle_request || large_angle_error || accel_moving || descent_not_demanded) {
+        bool use_mix_max = large_angle_request || large_angle_error || accel_moving || descent_not_demanded;
+
+        /*
+          special case for auto landing, we want a high degree of
+          attitude control until LAND_FINAL
+         */
+        if (in_vtol_land_sequence()) {
+            use_mix_max = !in_vtol_land_final();
+        }
+
+        if (use_mix_max) {
             attitude_control->set_throttle_mix_max(1.0);
         } else {
             attitude_control->set_throttle_mix_min();