Attach (recommended) or Link to PDF file here:
https://guichet.public.lu/dam-assets/catalogue-formulaires/tva/tva-option-aic-pers-morales/tva-option-aic-pers-morale-DE.pdf

Configuration:
- Web browser and its version: Firefox Nightly 91.0a1 (2021-06-26) (64-bit)
- Operating system and its version: Windows 10.0.19043.1052
- PDF.js version: N/A

Steps to reproduce the problem:
1. enableXFA in Firefox Nightly
2. Open form

What is the expected behavior? (add screenshot)
Header image is scaled properly
![image](https://user-images.githubusercontent.com/7104454/123525954-905d1f00-d6a2-11eb-840e-be30eee395e1.png)

What went wrong? (add screenshot)
Image is scaled to fit
![image](https://user-images.githubusercontent.com/7104454/123525957-96530000-d6a2-11eb-9991-bf7f37c6f1a5.png)


const edges = this.edge.children.slice();
    if (edges.length < 4) {
      const defaultEdge = edges[edges.length - 1] || new Edge({});
      for (let i = edges.length; i < 4; i++) {
        edges.push(defaultEdge);
      }
    }
    const edgeStyles = edges.map(node => {
      const style = node[$toStyle]();
      style.color = style.color || "#000000";
      return style;
    });
    const widths = edges.map(edge => edge.thickness);
    const insets = [0, 0, 0, 0];
    if (this.margin) {
      insets[0] = this.margin.topInset;
      insets[1] = this.margin.rightInset;
      insets[2] = this.margin.bottomInset;
      insets[3] = this.margin.leftInset;
    }
    this[$extra] = { widths, insets };
    
    const style = Object.create(null);
    if (this.margin) {
      Object.assign(style, this.margin[$toStyle]());
    }
    if (this.fill) {