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.
this.lineX = 0;
    this.lineY = 0;
    
    this.charSpacing = 0;
    this.wordSpacing = 0;
    this.textHScale = 1;
    this.textRenderingMode = TextRenderingMode.FILL;
    this.textRise = 0;
    
    this.fillColor = "#000000";
    this.strokeColor = "#000000";
    this.patternFill = false;
    
    this.fillAlpha = 1;
    this.strokeAlpha = 1;
    this.lineWidth = 1;
    this.activeSMask = null;
    this.transferMaps = null;
    this.startNewPathAndClipBox([0, 0, width, height]);
  }
  clone() {
    const clone = Object.create(this);
    clone.clipBox = this.clipBox.slice();
    return clone;
  }
  setCurrentPoint(x, y) {
    this.x = x;
    this.y = y;
  }
  updatePathMinMax(transform, x, y) {