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("margin");
    const size = measureToString(this.size);
    style.width = style.height = size;
    let type;
    let className;
    let groupId;
    const field = this[$getParent]()[$getParent]();
    const items =
      (field.items.children.length &&
        field.items.children[0][$toHTML]().html) ||
      [];
    const exportedValue = {
      on: (items[0] || "on").toString(),
      off: (items[1] || "off").toString(),
    };
    const value = (field.value && field.value[$text]()) || "off";
    const checked = value === exportedValue.on || undefined;
    const container = field[$getParent]();
    const fieldId = field[$uid];
    let dataId;
    if (container instanceof ExclGroup) {
      groupId = container[$uid];
      type = "radio";
      className = "xfaRadio";
      dataId = container[$data] && container[$data][$uid];