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):

"etched",
      "lowered",
      "raised",
    ]);
    
    this.thickness = Math.max(
      1,
      Math.round(getMeasurement(attributes.thickness, "0.5pt"))
    );
    this.use = attributes.use || "";
    this.usehref = attributes.usehref || "";
    this.color = null;
    this.extras = null;
  }
  [$toStyle]() {
    
    const style = toStyle(this, "visibility");
    Object.assign(style, {
      linecap: this.cap,
      width: measureToString(Math.max(1, Math.round(this.thickness))),
      color: this.color ? this.color[$toStyle]() : "#000000",
      style: "",
    });
    if (this.presence !== "visible") {
      style.style = "none";
    } else {
      switch (this.stroke) {
        case "solid":
          style.style = "solid";
          break;