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):

</pageSet>
      <subform name="first">
      </subform>
      <subform name="second">
        <breakBefore targetType="pageArea" startNew="1"/>
      </subform>
    </subform>
  </template>
  <xfa:datasets xmlns:xfa="http://www.xfa.org/schema/xfa-data/1.0/">
    <xfa:data>
    </xfa:data>
  </xfa:datasets>
</xdp:xdp>
      `;
      const factory = new XFAFactory({ "xdp:xdp": xml });
      expect(factory.numberPages).toEqual(2);
      const pages = factory.getPages();
      const page1 = pages.children[0];
      expect(page1.attributes.style).toEqual({
        height: "789px",
        width: "456px",
      });
      expect(page1.children.length).toEqual(2);
      const container = page1.children[0];
      expect(container.attributes.class).toEqual("xfaContentarea");
      expect(container.attributes.style).toEqual({
        height: "789px",
        width: "456px",
        left: "123px",
        top: "0px",