通过资产系统加载 gltf 模型

<a-scene>
  <a-assets>
    <a-asset-item id="duck" src="https://cdn.rawgit.com/KhronosGroup/glTF-Sample-Models/9176d098/1.0/Duck/glTF/Duck.gltf"></a-asset-item>
  </a-assets>

  <a-entity gltf-model="#duck" position="0 0 -5"></a-entity>
</a-scene>