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 (data[pos - lineSize] !== data[pos]) {
      points[j0] = data[pos] ? 1 : 8;
      ++count;
    }
    
    
    let sum = (data[pos] ? 4 : 0) + (data[pos - lineSize] ? 8 : 0);
    for (j = 1; j < width; j++) {
      sum =
        (sum >> 2) +
        (data[pos + 1] ? 4 : 0) +
        (data[pos - lineSize + 1] ? 8 : 0);
      if (POINT_TYPES[sum]) {
        points[j0 + j] = POINT_TYPES[sum];
        ++count;
      }
      pos++;
    }
    if (data[pos - lineSize] !== data[pos]) {
      points[j0 + j] = data[pos] ? 2 : 4;
      ++count;
    }
    if (count > POINT_TO_PROCESS_LIMIT) {
      return null;
    }
  }
  pos = lineSize * (height - 1);
  j0 = i * width1;
  if (data[pos] !== 0) {
    points[j0] = 8;