ArduPlane: Fix bug in forward throttle voltage compensation

The existing code does not adjust the upper and lower throttle limits with the result that at and of discharge climb and reverse thrust performance is significantly degraded.

ArduPlane/Plane.h

@@ -1042,7 +1042,7 @@ private:
     void servos_output(void);
     void servos_auto_trim(void);
     void servos_twin_engine_mix();
-    void throttle_voltage_comp();
+    void throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle);
     void throttle_watt_limiter(int8_t &min_throttle, int8_t &max_throttle);
     void throttle_slew_limit(SRV_Channel::Aux_servo_function_t func);
     bool suppress_throttle(void);

ArduPlane/servos.cpp

@@ -345,7 +345,7 @@ void Plane::set_servos_manual_passthrough(void)
 /*
   Scale the throttle to conpensate for battery voltage drop
  */
-void Plane::throttle_voltage_comp()
+void Plane::throttle_voltage_comp(int8_t &min_throttle, int8_t &max_throttle)
 {
     // return if not enabled, or setup incorrectly
     if (g2.fwd_thr_batt_voltage_min >= g2.fwd_thr_batt_voltage_max || !is_positive(g2.fwd_thr_batt_voltage_max)) {
@@ -370,6 +370,10 @@ void Plane::throttle_voltage_comp()
     // Ratio = 1 when voltage = voltage max, ratio increases as voltage drops
     const float ratio = g2.fwd_thr_batt_voltage_max / batt_voltage_resting_estimate;
 
+    // Scale the throttle limits to prevent subsequent clipping
+    min_throttle = MAX((int8_t)(ratio * (float)min_throttle), -100);
+    max_throttle = MIN((int8_t)(ratio * (float)max_throttle),  100);
+
     SRV_Channels::set_output_scaled(SRV_Channel::k_throttle,
                                         constrain_int16(SRV_Channels::get_output_scaled(SRV_Channel::k_throttle) * ratio, -100, 100));
 }
@@ -455,7 +459,7 @@ void Plane::set_servos_controlled(void)
     }
 
     // conpensate for battery voltage drop
-    throttle_voltage_comp();
+    throttle_voltage_comp(min_throttle, max_throttle);
 
     // apply watt limiter
     throttle_watt_limiter(min_throttle, max_throttle);