AC_AttitudeControl: added get_throttle_mix()

and when we use set_throttle_mix_value(), set both desired and actual

thanks to Leonard for the suggestions

libraries/AC_AttitudeControl/AC_AttitudeControl.h

@@ -247,6 +247,7 @@ public:
     virtual void set_throttle_mix_man() {}
     virtual void set_throttle_mix_max() {}
     virtual void set_throttle_mix_value(float value) {}
+    virtual float get_throttle_mix(void) const { return 0; }
 
     // enable use of flybass passthrough on heli
     virtual void use_flybar_passthrough(bool passthrough, bool tail_passthrough) {}

libraries/AC_AttitudeControl/AC_AttitudeControl_Multi.h

@@ -66,7 +66,8 @@ public:
     void set_throttle_mix_min() override { _throttle_rpy_mix_desired = _thr_mix_min; }
     void set_throttle_mix_man() override { _throttle_rpy_mix_desired = _thr_mix_man; }
     void set_throttle_mix_max() override { _throttle_rpy_mix_desired = _thr_mix_max; }
-    void set_throttle_mix_value(float value) override { _throttle_rpy_mix_desired = value; }
+    void set_throttle_mix_value(float value) override { _throttle_rpy_mix_desired = _throttle_rpy_mix = value; }
+    float get_throttle_mix(void) const override { return _throttle_rpy_mix; }
 
     // are we producing min throttle?
     bool is_throttle_mix_min() const override { return (_throttle_rpy_mix < 1.25f*_thr_mix_min); }