cesium流动管线

以下是两种实现cesium流动管线的方式：
1. 使用entity方式[^1]：

// 创建entity
var entity = viewer.entities.add({
  polyline: {
    positions: Cesium.Cartesian3.fromDegreesArray([
      -75, 35,
      -125, 35
    ]),
    width: 10,
    material: new Cesium.PolylineFlowMaterialProperty({
      color: Cesium.Color.RED,
      duration: 3000
    })
  }
});

2. 使用内置材质类型：

// 创建primitive
var primitive = viewer.scene.primitives.add(new Cesium.Primitive({
  geometryInstances: new Cesium.GeometryInstance({
    geometry: new Cesium.PolylineGeometry({
      positions: Cesium.Cartesian3.fromDegreesArray([
        -75, 35,
        -125, 35
      ]),
      width: 10
    })
  }),
  appearance: new Cesium.FlowAppearance({
    color: Cesium.Color.RED,
    duration: 3000
  })
}));