Copter: use fast tan for accel to lean angle calcs

Committed on behalf of Leonard

libraries/AC_WPNav/AC_WPNav.cpp

@@ -585,8 +585,8 @@ void AC_WPNav::get_loiter_acceleration_to_lean_angles(float accel_lat, float acc
     accel_right = -accel_lat*_sin_yaw + accel_lon*_cos_yaw;
 
     // update angle targets that will be passed to stabilize controller
-    _desired_roll = constrain_float((accel_right*_cos_pitch/(-z_accel_meas))*(18000/M_PI), -_lean_angle_max, _lean_angle_max);
-    _desired_pitch = constrain_float((-accel_forward/(-z_accel_meas))*(18000/M_PI), -_lean_angle_max, _lean_angle_max);
+    _desired_roll = constrain_float(fast_atan(accel_right*_cos_pitch/(-z_accel_meas))*(18000/M_PI), -_lean_angle_max, _lean_angle_max);
+    _desired_pitch = constrain_float(fast_atan(-accel_forward/(-z_accel_meas))*(18000/M_PI), -_lean_angle_max, _lean_angle_max);
 }
 
 // get_bearing_cd - return bearing in centi-degrees between two positions