framebuffer not ready

```
g3d_test() {
/*获取元素引用*/
var ganvas = this.$refs["gcanva"];
/*通过元素引用获取canvas对象*/
var canvaObj = enable(ganvas, {
bridge: WeexBridge
});
console.log(canvaObj)
console.log(G3D.Engine);

const engine = new G3D.Engine(canvaObj);
console.log(engine)

const scene = new G3D.Scene(engine);
console.log(scene)
},
```

Begin at the g3d_test entry point, especially the enable(ganvas, { bridge: WeexBridge }) call and G3D.Engine construction shown in the issue. Reproduce the framebuffer-not-ready state and trace initialization through Scene creation; done means the supplied setup reaches a ready framebuffer without the reported failure.

Written by the indexing model from the issue text.