This is a regression from #12864. The underscore is now too thin when the document is zoomed in.

URL: https://github.com/mozilla/pdf.js/files/2321540/issue6127.pdf
PDF.js 2.7.526: https://i.imgur.com/T4mIpct.png
PDF.js 2.7.510: https://i.imgur.com/nbwEpGT.png
Adobe Acrobat Reader DC 2020.013.20074: https://i.imgur.com/wDFBf3R.png

The screenshots were taken at 400% zoom.
case "RI":
          this.setRenderingIntent(value);
          break;
        case "FL":
          this.setFlatness(value);
          break;
        case "Font":
          this.setFont(value[0], value[1]);
          break;
        case "CA":
          this.current.strokeAlpha = state[1];
          break;
        case "ca":
          this.current.fillAlpha = state[1];
          this.ctx.globalAlpha = state[1];
          break;
        case "BM":
          this.ctx.globalCompositeOperation = value;
          break;
        case "SMask":
          this.current.activeSMask = value ? this.tempSMask : null;
          this.tempSMask = null;
          this.checkSMaskState();
          break;
        case "TR":
          this.current.transferMaps = value;
      }
    }
  }
  checkSMaskState() {