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 pageAreas = root.pageSet.pageArea.children;
    const mainHtml = {
      name: "div",
      children: [],
    };
    let pageArea = null;
    let breakBefore = null;
    let breakBeforeTarget = null;
    if (root.breakBefore.children.length >= 1) {
      breakBefore = root.breakBefore.children[0];
      breakBeforeTarget = breakBefore.target;
    } else if (
      root.subform.children.length >= 1 &&
      root.subform.children[0].breakBefore.children.length >= 1
    ) {
      breakBefore = root.subform.children[0].breakBefore.children[0];
      breakBeforeTarget = breakBefore.target;
    } else if (root.break && root.break.beforeTarget) {
      breakBefore = root.break;
      breakBeforeTarget = breakBefore.beforeTarget;
    } else if (
      root.subform.children.length >= 1 &&
      root.subform.children[0].break &&
      root.subform.children[0].break.beforeTarget
    ) {
      breakBefore = root.subform.children[0].break;
      breakBeforeTarget = breakBefore.beforeTarget;
    }
    if (breakBefore) {
      const target = this[$searchNode](