AP_Motors: add get roll factor

libraries/AP_Motors/AP_MotorsMatrix.h

@@ -51,6 +51,10 @@ public:
     // return number of motor that has failed.  Should only be called if get_thrust_boost() returns true
     uint8_t             get_lost_motor() const override { return _motor_lost_index; }
 
+    // return the roll factor of any motor, this is used for tilt rotors and tail sitters
+    // using copter motors for forward flight
+    float               get_roll_factor(uint8_t i) override { return _roll_factor[i]; }
+
 protected:
     // output - sends commands to the motors
     void                output_armed_stabilizing() override;

libraries/AP_Motors/AP_Motors_Class.h

@@ -157,6 +157,10 @@ public:
     // set loop rate. Used to support loop rate as a parameter
     void                set_loop_rate(uint16_t loop_rate) { _loop_rate = loop_rate; }
 
+    // return the roll factor of any motor, this is used for tilt rotors and tail sitters
+    // using copter motors for forward flight
+    virtual float       get_roll_factor(uint8_t i) { return 0.0f; }
+
     enum pwm_type { PWM_TYPE_NORMAL     = 0,
                     PWM_TYPE_ONESHOT    = 1,
                     PWM_TYPE_ONESHOT125 = 2,