mars3d实现GraphicLayer获取当前相机视角内的可视点位（矢量数据

效果：

mars3d实现GraphicLayer获取当前相机视角内的可视点位（矢量数据

相关依赖api：

1. map.getExtent();

提取地球当前视域边界,示例：{ xmin: 70, xmax: 140, ymin: 0, ymax: 55, height: 0, }

2.graphicLayer.eachGraphic遍历所有矢量数据并将其作为参数传递给回调函数判断是否在当前视域边界内

3.写个判断：

/**
* 获取地图边界内的矢量点位
* @param graphicLayer
* @param isExtend
*/
const getGraphics = (graphicLayer: mars3d.layer.GraphicLayer | undefined, isExtend: Boolean = true) => {
if (graphicLayer && (graphicLayer as any)._map) {
const map = (graphicLayer as any)._map as mars3d.Map;
const graphics: mars3d.graphic.BillboardEntity[] = [];
const extend = map.getExtent();
if (isExtend) {
graphicLayer.eachGraphic((graphic: mars3d.graphic.BillboardEntity) => {
const { lng, lat } = graphic.point;
if (lng >= extend.xmin && lng = extend.ymin && lat