fix: position

[bobby169]

比如想用popup做从中间bounce-in跳出的弹窗，则可设置popupTransition为popup-bounce等自定义

.popup-bounce-enter-active {
    animation: popup-bounce-in .5s;
  }
  .popup-bounce-leave-active {
    animation: popup-bounce-in .5s reverse;
  }
  @keyframes popup-bounce-in {
    0% {
      transform: translate3d(-50%, -50%, 0) scale(0);
    }
    50% {
      transform: translate3d(-50%, -50%, 0) scale(1.2);
    }
    100% {
      transform: translate3d(-50%, -50%, 0) scale(1);
    }
  }