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)


if (!("message" in e)) {
        return JSON.stringify(e);
      }
      return e.message + ("stack" in e ? " at " + e.stack.split("\n")[0] : "");
    },
    _getLastPageNumber: function Driver_getLastPageNumber(task) {
      if (!task.pdfDoc) {
        return task.firstPage || 1;
      }
      var lastPageNumber = task.lastPage || 0;
      if (!lastPageNumber || lastPageNumber > task.pdfDoc.numPages) {
        lastPageNumber = task.pdfDoc.numPages;
      }
      return lastPageNumber;
    },
    _nextPage: function Driver_nextPage(task, loadError) {
      var self = this;
      var failure = loadError || "";
      var ctx;
      if (!task.pdfDoc) {
        var dataUrl = this.canvas.toDataURL("image/png");
        this._sendResult(dataUrl, task, failure, function () {
          self._log(
            "done" + (failure ? " (failed !: " + failure + ")" : "") + "\n"
          );
          self.currentTask++;
          self._nextTask();
        });
        return;
      }