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)


return renderTask.promise.then(function () {
                    completeRender(false);
                  });
                })
                .catch(function (error) {
                  completeRender("render : " + error);
                });
            },
            function (error) {
              self._snapshot(task, "render : " + error);
            }
          );
        } catch (e) {
          failure = "page setup : " + this._exceptionToString(e);
          this._snapshot(task, failure);
        }
      }
    },
    _clearCanvas: function Driver_clearCanvas() {
      var ctx = this.canvas.getContext("2d", { alpha: false });
      ctx.beginPath();
      ctx.clearRect(0, 0, this.canvas.width, this.canvas.height);
    },
    _snapshot: function Driver_snapshot(task, failure) {
      var self = this;
      this._log("Snapshotting... ");
      var dataUrl = this.canvas.toDataURL("image/png");
      this._sendResult(dataUrl, task, failure, function () {
        self._log(
          "done" + (failure ? " (failed !: " + failure + ")" : "") + "\n"