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.
}
      if (heightScale > 2 && paintHeight > 1) {
        newHeight = Math.ceil(paintHeight / 2);
        heightScale /= paintHeight / newHeight;
      }
      tmpCanvas = this.cachedCanvases.getCanvas(
        tmpCanvasId,
        newWidth,
        newHeight
      );
      tmpCtx = tmpCanvas.context;
      tmpCtx.clearRect(0, 0, newWidth, newHeight);
      tmpCtx.drawImage(
        img,
        0,
        0,
        paintWidth,
        paintHeight,
        0,
        0,
        newWidth,
        newHeight
      );
      img = tmpCanvas.canvas;
      paintWidth = newWidth;
      paintHeight = newHeight;
      tmpCanvasId = tmpCanvasId === "prescale1" ? "prescale2" : "prescale1";
    }
    return {
      img,