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):

this.handler = new XFAObjectArray();
    this.hyphenation = new XFAObjectArray();
    this.image = new XFAObjectArray();
    this.imageEdit = new XFAObjectArray();
    this.integer = new XFAObjectArray();
    this.issuers = new XFAObjectArray();
    this.items = new XFAObjectArray();
    this.keep = new XFAObjectArray();
    this.keyUsage = new XFAObjectArray();
    this.line = new XFAObjectArray();
    this.linear = new XFAObjectArray();
    this.lockDocument = new XFAObjectArray();
    this.manifest = new XFAObjectArray();
    this.margin = new XFAObjectArray();
    this.mdp = new XFAObjectArray();
    this.medium = new XFAObjectArray();
    this.message = new XFAObjectArray();
    this.numericEdit = new XFAObjectArray();
    this.occur = new XFAObjectArray();
    this.oid = new XFAObjectArray();
    this.oids = new XFAObjectArray();
    this.overflow = new XFAObjectArray();
    this.pageArea = new XFAObjectArray();
    this.pageSet = new XFAObjectArray();
    this.para = new XFAObjectArray();
    this.passwordEdit = new XFAObjectArray();
    this.pattern = new XFAObjectArray();
    this.picture = new XFAObjectArray();
    this.radial = new XFAObjectArray();
    this.reason = new XFAObjectArray();