Use explicit delta from buffer

src/processor/processor_imu.cpp

@@ -343,7 +343,7 @@ VectorXd ProcessorImu::bootstrapDelta() const
     }
     // now compose with delta in last_ptr_
     dt                = last_ptr_->getBuffer().back().ts_ - origin_ptr_->getTimeStamp();
-    const auto& delta = last_ptr_->getDeltaPreint();
+    const auto& delta = last_ptr_->getBuffer().back().delta_integr_;
     delta_int         = imu::compose(delta_int, delta, dt);
 
     return delta_int;