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)


return this[$extra].availableSpace;
  }
  [$toHTML](availableSpace) {
    
    const style = toStyle(this, "position");
    const attributes = {
      style,
      id: this[$uid],
      class: ["xfaArea"],
    };
    if (isPrintOnly(this)) {
      attributes.class.push("xfaPrintOnly");
    }
    if (this.name) {
      attributes.xfaName = this.name;
    }
    const children = [];
    this[$extra] = {
      children,
      width: 0,
      height: 0,
      availableSpace,
    };
    const result = this[$childrenToHTML]({
      filter: new Set([
        "area",