背景水印生成

需求说明：页面增加背景水印

解决方案：前端生成水印背景图片（此借用canvas的toDataUrl方法转为base64）

   /**
    * 为网页添加文字水印
    * @param {String} str 要添加的字符串
    */
    addWaterMarker(str) {
      const can = document.createElement('canvas')
      const content = this.$refs.bgReport
      content.appendChild(can)
      can.width = 200
      can.height = 200
      var cans = can.getContext('2d')
      cans.rotate(-20 * Math.PI / 180)
      cans.font = "16px Microsoft JhengHei"
      cans.fillStyle = "rgba(17, 17, 17, 0.50)"
      cans.textAlign = 'left'
      cans.textBaseline = 'Middle'
      cans.fillText(str, can.width / 3, can.height / 2)
      content.style.backgroundImage = "url(" + can.toDataURL("image/png") + ")"
    },