If there are remaining catpures in the tracker queue after the tracker is shutdown, k4abt_tracker_pop_result() can still return successfully. Once the tracker queue is empty, the k4abt_tracker_pop_result() call will always immediately return failure.
This function may be called while another thread is blocking in k4abt_tracker_enqueue_capture() or k4abt_tracker_pop_result(). Calling this function while another thread is in that function will result in that function returning a failure.