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.
let type = points[p];
    do {
      const step = steps[type];
      do {
        p += step;
      } while (!points[p]);
      const pp = points[p];
      if (pp !== 5 && pp !== 10) {
        
        type = pp;
        
        points[p] = 0;
      } else {
        
        
        type = pp & ((0x33 * type) >> 4);
        
        points[p] &= (type >> 2) | (type << 2);
      }
      coords.push(p % width1, (p / width1) | 0);
      if (!points[p]) {
        --count;
      }
    } while (p0 !== p);
    outlines.push(coords);
    --i;
  }
  const drawOutline = function (c) {
    c.save();