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):

defaultValue: 0,
      validate: x => x === 1,
    });
    this.rowColumnRatio = getRatio(attributes.rowColumnRatio);
    this.startChar = attributes.startChar || "";
    this.textLocation = getStringOption(attributes.textLocation, [
      "below",
      "above",
      "aboveEmbedded",
      "belowEmbedded",
      "none",
    ]);
    this.truncate = getInteger({
      data: attributes.truncate,
      defaultValue: 0,
      validate: x => x === 1,
    });
    this.type = getStringOption(
      attributes.type ? attributes.type.toLowerCase() : "",
      [
        "aztec",
        "codabar",
        "code2of5industrial",
        "code2of5interleaved",
        "code2of5matrix",
        "code2of5standard",
        "code3of9",
        "code3of9extended",
        "code11",
        "code49",