Plane: reset rate controller integrators if not run for 2s

this prevents a bug where integrator buildup can happen from a
previous flight mode

ArduPlane/Attitude.cpp

@@ -370,7 +370,17 @@ void Plane::stabilize()
         nav_pitch_cd = constrain_float((quadplane.tailsitter.transition_angle+5)*100, 5500, 8500),
         nav_roll_cd = 0;
     }
-    
+
+    uint32_t now = AP_HAL::millis();
+    if (now - last_stabilize_ms > 2000) {
+        // if we haven't run the rate controllers for 2 seconds then
+        // reset the integrators
+        rollController.reset_I();
+        pitchController.reset_I();
+        yawController.reset_I();
+    }
+    last_stabilize_ms = now;
+
     if (control_mode == &mode_training) {
         stabilize_training(speed_scaler);
     } else if (control_mode == &mode_acro) {

ArduPlane/Plane.h

@@ -349,6 +349,9 @@ private:
     // This is used to enable the inverted flight feature
     bool inverted_flight;
 
+    // last time we ran roll/pitch stabilization
+    uint32_t last_stabilize_ms;
+    
     // Failsafe
     struct {
         // Used to track if the value on channel 3 (throtttle) has fallen below the failsafe threshold