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):

]);
    if (this.layout.includes("row")) {
      const columnWidths = this[$getParent]().columnWidths;
      if (Array.isArray(columnWidths) && columnWidths.length > 0) {
        this[$extra].columnWidths = columnWidths;
        this[$extra].currentColumn = 0;
      }
    }
    const style = toStyle(
      this,
      "anchorType",
      "dimensions",
      "position",
      "presence",
      "borderMarginPadding",
      "hAlign"
    );
    const classNames = ["xfaSubform"];
    const cl = layoutClass(this);
    if (cl) {
      classNames.push(cl);
    }
    attributes.style = style;
    attributes.class = classNames.join(" ");
    if (this.name) {
      attributes.xfaName = this.name;
    }
    let failure;
    if (this.layout === "lr-tb" || this.layout === "rl-tb") {
      for (