a real screen shot by JS
forget htmltocanvas
forget dom to canvas
忘记htmltocanvas 忘掉通过dom转canvas进行截图，既费劲，有bug又不好用，还会丢失样式和 字体，真正的所见即所得的截屏
通过navigator.mediaDevices.getDisplayMedia 将屏幕分享，然后投射到video，然后video绘制canvas实现真实截屏效果。
navigator.mediaDevices.getDisplayMedia
// Down load the screenShot.js or copy into your js
// import
import {screenCapture} from '../js/screenShot.js'
// prepare dom and video
let Dom = document.querySelector("#demo-area-to-shot")
let DomHeight = Dom.offsetHeight;
let DomWidth = Dom.offsetWidth;
let videoDom = document.querySelector("#testVideo")
// screenShot
screenCapture(videoDom,DomWidt,DomHeight)
欢迎学习交流！