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.
while (this.stateStack.length || this.current.activeSMask !== null) {
      this.restore();
    }
    this.ctx.restore();
    if (this.transparentCanvas) {
      this.ctx = this.compositeCtx;
      this.ctx.save();
      this.ctx.setTransform(1, 0, 0, 1, 0, 0); 
      this.ctx.drawImage(this.transparentCanvas, 0, 0);
      this.ctx.restore();
      this.transparentCanvas = null;
    }
    this.cachedCanvases.clear();
    this.cachedPatterns.clear();
    if (this.imageLayer) {
      this.imageLayer.endLayout();
    }
  }
  _scaleImage(img, inverseTransform) {