CustomPrimitive移动后偶尔会消失不见的问题

http://earthsdk.com/v/last/Apps/Examples/?menu=false&url=./earth-customPrimitive-circle.html
在148行后插入 下面代码也能复现问题。

```
let viewer = earth._viewer;
let handler = new Cesium.ScreenSpaceEventHandler(viewer.canvas);
handler.setInputAction(event => {
var cartographic = earth.pickPosition(event.endPosition);
p.position = cartographic;
console.log(p.show);
console.log(p.position);
}, Cesium.ScreenSpaceEventType.MOUSE_MOVE);

let startTime;
handler.setInputAction(event => {
console.log('left_down')
console.log(event)
startTime = new Date();
}, Cesium.ScreenSpaceEventType.LEFT_DOWN);

let endTime;
handler.setInputAction(event => {
console.log('left_up')
console.log(event)
endTime = new Date();
console.log(startTime);
console.log(endTime);
console.log(endTime - startTime);
if(endTime - startTime < 200){
handler.removeInputAction( Cesium.ScreenSpaceEventType.MOUSE_MOVE)
handler.removeInputAction( Cesium.ScreenSpaceEventType.LEFT_DOWN)
handler.removeInputAction( Cesium.ScreenSpaceEventType.LEFT_UP)
}
}, Cesium.ScreenSpaceEventType.LEFT_UP);
```

Open the linked earth-customPrimitive-circle.html example and inspect the code around line 148, then reproduce the issue using the supplied Cesium screen-space event handlers. Trace the CustomPrimitive position update during mouse movement and verify when the primitive disappears. Done means moving the CustomPrimitive no longer intermittently makes it invisible.

Written by the indexing model from the issue text.