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 getAvailableSpace(this);
  }
  [$isSplittable](x) {
    
    
    const root = this[$getTemplateRoot]();
    const contentArea = root[$extra].currentContentArea;
    if (contentArea && Math.max(this.minH, this.h || 0) >= contentArea.h) {
      return true;
    }
    if (this.layout === "position") {
      return false;
    }
    if (this.keep && this.keep.intact !== "none") {
      return false;
    }
    const parentLayout = this[$getParent]().layout;
    if (parentLayout && parentLayout.includes("row")) {
      return false;
    }
    if (this.overflow && this.overflow.target) {
      const target = root[$searchNode](this.overflow.target, this);
      return target && target[0] === contentArea;
    }
    return true;
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  [$toHTML](availableSpace) {
    if (this.break) {