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.
const length = bytes.length;
  for (let i = 3; i < length; i += 4) {
    const alpha = bytes[i];
    if (alpha === 0) {
      bytes[i - 3] = r0;
      bytes[i - 2] = g0;
      bytes[i - 1] = b0;
    } else if (alpha < 255) {
      const alpha_ = 255 - alpha;
      bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) >> 8;
      bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) >> 8;
      bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) >> 8;
    }
  }
}
function composeSMaskAlpha(maskData, layerData, transferMap) {
  const length = maskData.length;
  const scale = 1 / 255;
  for (let i = 3; i < length; i += 4) {
    const alpha = transferMap ? transferMap[maskData[i]] : maskData[i];
    layerData[i] = (layerData[i] * alpha * scale) | 0;
  }
}
function composeSMaskLuminosity(maskData, layerData, transferMap) {