Cesium加载带有背景图（包含多个背景图，图标和文字并排）的billboard

思路：如果是简单的背景加文字，背景图会把文字挡住，又或者是背景图要有图标加文字，这时需要用canvas将背景图，图标和文字合并成一张图片，加载billboard的时候直接加载合成之后的图片即可。

let canvas = document.createElement("canvas");
canvas.width = 100;
canvas.height = 30;
let context = canvas.getContext("2d");
context.rect(0, 0, canvas.width, canvas.height);
let bgImg = new Image();
bgImg.src = require("../img/bg.png"); // 背景图的url
bgImg.crossOrigin = "Anonymous";
bgImg.onload = () => {
    context.fillStyle = '#ffffff'; // 设置文字的填充颜色
    context.font = 'italic 15px Georgia'; // 设置文字的填充样式
    context.fillText("文本内容", 10, 20);
    context.drawImage(bgImg, 0, 0, 100, 30);
    let img = new Image();
    img.src = require("../img/icon.png");; // 需要合进去的图片url
    img.crossOrigin = "Anonymous";
    img.onload = () => {
        context.drawImage(img, 0, 0, 16, 16);
        let base64 = canvas.toDataURL("image/png");
        this.viewer.entities.add({
            position: Cesium.Cartesian3.fromDegrees(Number(lng), Number(lat), 0),
            billboard: {
                  image: base64,
                  heightReference: Cesium.HeightReference.CLAMP_TO_GROUND,
                  verticalOrigin: Cesium.VerticalOrigin.BOTTOM,
                  disableDepthTestDistance: Number.POSITIVE_INFINITY,
                  pixelOffset: new Cesium.Cartesian3(0, 0),   //偏移量
            }
        });
    };    
}