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):

margins = margins || [0, 0, 0, 0];
    const edgeStyles = edges.map(node => {
      const style = node[$toStyle]();
      style.color = style.color || "#000000";
      return style;
    });
    let style;
    if (this.margin) {
      style = this.margin[$toStyle]();
      margins[0] = this.margin.topInset;
      margins[1] = this.margin.rightInset;
      margins[2] = this.margin.bottomInset;
      margins[3] = this.margin.leftInset;
    } else {
      style = Object.create(null);
    }
    let isForUi = false;
    const parent = this[$getParent]();
    const grandParent = parent ? parent[$getParent]() : null;
    if (grandParent instanceof Ui) {
      isForUi = true;
    }
    if (this.fill) {
      Object.assign(style, this.fill[$toStyle]());
    }
    let hasRadius = false;
    if (this.corner.children.some(node => node.radius !== 0)) {
      const cornerStyles = this.corner.children.map(node => node[$toStyle]());
      if (cornerStyles.length === 2 || cornerStyles.length === 3) {
        const last = cornerStyles[cornerStyles.length - 1];