屏幕全屏

在項目中，會有全屏的功能：

data() {
    return {
        fullscreen: false,
    }
},

// methods
changeFullScreen() {
     // 全屏事件
    let element = document.documentElement;
    if (this.fullscreen) {
        if (document.exitFullscreen) {
            document.exitFullscreen();
        } else if (document.webkitCancelFullScreen) {
            document.webkitCancelFullScreen();
        } else if (document.mozCancelFullScreen) {
            document.mozCancelFullScreen();
        } else if (document.msExitFullscreen) {
            document.msExitFullscreen();
        }
    } else {
        if (element.requestFullscreen) {
            element.requestFullscreen();
        } else if (element.webkitRequestFullScreen) {
            element.webkitRequestFullScreen();
        } else if (element.mozRequestFullScreen) {
            element.mozRequestFullScreen();
        } else if (element.msRequestFullscreen) {
            // IE11
            element.msRequestFullscreen();
        }
    }
    this.fullscreen = !this.fullscreen;
}