Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Firefox 88.0.1
- Operating system and its version: Windows 10.0.19043.985]
- PDF.js version: (commit 706c27b9d4b3b9ff6fc3de82bcf71a0610604a1c) (also tested pr/13473 at 296bbdc4)
- Is a browser extension: N/A

Steps to reproduce the problem:
1. Enable experimental XFA
2. Open PDF
[dir2020-02_form.pdf](https://github.com/mozilla/pdf.js/files/6571084/dir2020-02_form.pdf)

What is the expected behavior? (add screenshot)
Margins of `subforms` (e.g. *General_Election_Information*, *General_Information_Other*) are honored.
![image](https://user-images.githubusercontent.com/7104454/120212605-32066300-c200-11eb-9c66-f2d05c9451ec.png)
What went wrong? (add screenshot)
Margins are not honored. 
![image](https://user-images.githubusercontent.com/7104454/120212535-1ac77580-c200-11eb-8332-9924602ef991.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

extra.line.children.push(html);
      if (extra.attempt === 0) {
        
        const [, , w, h] = bbox;
        extra.currentWidth += w;
        extra.height = Math.max(extra.height, extra.prevHeight + h);
      } else {
        const [, , w, h] = bbox;
        extra.width = Math.max(extra.width, extra.currentWidth);
        extra.currentWidth = w;
        extra.prevHeight = extra.height;
        extra.height += h;
        
        extra.attempt = 0;
      }
      break;
    case "rl-row":
    case "row": {
      extra.children.push(html);
      const [, , w, h] = bbox;
      extra.width += w;
      extra.height = Math.max(extra.height, h);
      const height = measureToString(extra.height);
      for (const child of extra.children) {
        if (child.attributes.class === "xfaWrapper") {
          child.children[child.children.length - 1].attributes.style.height =
            height;
        } else {
          child.attributes.style.height = height;
        }