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.
currentCtx.scale(scaleX, scaleY);
      currentCtx.save();
    }
    
    
    copyCtxState(currentCtx, groupCtx);
    this.ctx = groupCtx;
    this.setGState([
      ["BM", "source-over"],
      ["ca", 1],
      ["CA", 1],
    ]);
    this.groupStack.push({
      ctx: currentCtx,
      suspendedCtx,
    });
    this.groupLevel++;
  }
  endGroup(group) {
    if (!this.contentVisible) {
      return;
    }
    this.groupLevel--;
    const groupCtx = this.ctx;
    const { ctx, suspendedCtx } = this.groupStack.pop();
    this.ctx = ctx;
    
    
    this.ctx.imageSmoothingEnabled = false;
    if (suspendedCtx) {