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):

const storage = parameters.annotationStorage;
    const root = parameters.xfa;
    const intent = parameters.intent;
    const rootHtml = document.createElement(root.name);
    if (root.attributes) {
      this.setAttributes(rootHtml, root);
    }
    const stack = [[root, -1, rootHtml]];
    const rootDiv = parameters.div;
    rootDiv.appendChild(rootHtml);
    let { viewport } = parameters;
    if (!(viewport instanceof PageViewport)) {
      viewport = new PageViewport(viewport);
    }
    const coeffs = viewport.transform.join(",");
    rootDiv.style.transform = `matrix(${coeffs})`;
    
    rootDiv.setAttribute("class", "xfaLayer xfaFont");
    while (stack.length > 0) {
      const [parent, i, html] = stack[stack.length - 1];
      if (i + 1 === parent.children.length) {
        stack.pop();
        continue;
      }
      const child = parent.children[++stack[stack.length - 1][1]];
      if (child === null) {
        continue;
      }
      const { name } = child;
      if (name === "#text") {