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.
imgData,
    scaleX,
    skewX = 0,
    skewY = 0,
    scaleY,
    positions
  ) {
    if (!this.contentVisible) {
      return;
    }
    const ctx = this.ctx;
    ctx.save();
    const currentTransform = ctx.mozCurrentTransform;
    ctx.transform(scaleX, skewX, skewY, scaleY, 0, 0);
    const mask = this._createMaskCanvas(imgData);
    ctx.setTransform(1, 0, 0, 1, 0, 0);
    for (let i = 0, ii = positions.length; i < ii; i += 2) {
      const trans = Util.transform(currentTransform, [
        scaleX,
        skewX,
        skewY,
        scaleY,
        positions[i],