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.
positions[i + 1],
      ]);
      const [x, y] = Util.applyTransform([0, 0], trans);
      ctx.drawImage(mask.canvas, x, y);
    }
    ctx.restore();
    this.compose();
  }
  paintImageMaskXObjectGroup(images) {
    if (!this.contentVisible) {
      return;
    }
    const ctx = this.ctx;
    const fillColor = this.current.fillColor;
    const isPatternFill = this.current.patternFill;
    for (let i = 0, ii = images.length; i < ii; i++) {
      const image = images[i];
      const width = image.width,
        height = image.height;
      const maskCanvas = this.cachedCanvases.getCanvas(
        "maskCanvas",
        width,
        height
      );
      const maskCtx = maskCanvas.context;
      maskCtx.save();
      putBinaryImageMask(maskCtx, image);
      maskCtx.globalCompositeOperation = "source-in";
      maskCtx.fillStyle = isPatternFill
        ? fillColor.getPattern(