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 (!this.contentVisible) {
      return;
    }
    const ctx = this.ctx;
    const w = imgData.width;
    const h = imgData.height;
    const tmpCanvas = this.cachedCanvases.getCanvas("inlineImage", w, h);
    const tmpCtx = tmpCanvas.context;
    putBinaryImageData(tmpCtx, imgData, this.current.transferMaps);
    for (let i = 0, ii = map.length; i < ii; i++) {
      const entry = map[i];
      ctx.save();
      ctx.transform.apply(ctx, entry.transform);
      ctx.scale(1, -1);
      ctx.drawImage(
        tmpCanvas.canvas,
        entry.x,
        entry.y,
        entry.w,
        entry.h,
        0,
        -1,
        1,
        1