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 style = toStyle(this, "border", "margin");
    const ui = this[$getParent]();
    const field = ui[$getParent]();
    const children = [];
    if (field.items.children.length > 0) {
      const items = field.items;
      let displayedIndex = 0;
      let saveIndex = 0;
      if (items.children.length === 2) {
        displayedIndex = items.children[0].save;
        saveIndex = 1 - displayedIndex;
      }
      const displayed = items.children[displayedIndex][$toHTML]().html;
      const values = items.children[saveIndex][$toHTML]().html;
      const value = (field.value && field.value[$text]()) || "";
      for (let i = 0, ii = displayed.length; i < ii; i++) {
        const option = {
          name: "option",
          attributes: {
            value: values[i] || displayed[i],
          },
          value: displayed[i],
        };
        if (values[i] === value) {