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)


useWorkerFetch: task.useWorkerFetch,
            enableXfa: task.enableXfa,
            styleElement: xfaStyleElement,
          });
          loadingTask.promise.then(
            doc => {
              if (task.enableXfa) {
                task.fontRules = "";
                for (const rule of xfaStyleElement.sheet.cssRules) {
                  task.fontRules += rule.cssText + "\n";
                }
              }
              task.pdfDoc = doc;
              task.optionalContentConfigPromise =
                doc.getOptionalContentConfig();
              this._nextPage(task, failure);
            },
            err => {
              failure = "Loading PDF document: " + err;
              this._nextPage(task, failure);
            }
          );
          return;
        } catch (e) {
          failure = "Loading PDF document: " + this._exceptionToString(e);
        }
        this._nextPage(task, failure);
      });
    },
    _cleanup() {