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.current.activeSMask) {
      return;
    }
    if (!dirtyBox) {
      dirtyBox = [0, 0, this.ctx.canvas.width, this.ctx.canvas.height];
    } else {
      dirtyBox[0] = Math.floor(dirtyBox[0]);
      dirtyBox[1] = Math.floor(dirtyBox[1]);
      dirtyBox[2] = Math.ceil(dirtyBox[2]);
      dirtyBox[3] = Math.ceil(dirtyBox[3]);
    }
    const smask = this.current.activeSMask;
    const suspendedCtx = this.suspendedCtx;
    composeSMask(suspendedCtx, smask, this.ctx, dirtyBox);
    
    
    this.ctx.save();
    this.ctx.setTransform(1, 0, 0, 1, 0, 0);
    this.ctx.clearRect(0, 0, this.ctx.canvas.width, this.ctx.canvas.height);
    this.ctx.restore();
  }
  save() {