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.
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      const m = this.ctx.mozCurrentTransform;
      const absDet = Math.abs(m[0] * m[3] - m[2] * m[1]);
      const sqNorm1 = m[0] ** 2 + m[2] ** 2;
      const sqNorm2 = m[1] ** 2 + m[3] ** 2;
      const pixelHeight = Math.sqrt(Math.max(sqNorm1, sqNorm2)) / absDet;
      if (sqNorm1 !== sqNorm2 && this._combinedScaleFactor * pixelHeight > 1) {
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        this._cachedGetSinglePixelWidth = -(
          this._combinedScaleFactor * pixelHeight
        );
      } else if (absDet > Number.EPSILON) {
        this._cachedGetSinglePixelWidth = pixelHeight;
      } else {
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        this._cachedGetSinglePixelWidth = 1;
      }
    }
    return this._cachedGetSinglePixelWidth;
  }
  getCanvasPosition(x, y) {