Ensure that the `sourceEventType` parameter is actually optional in `PDFViewerApplication.{download, save}` (PR 12248 follow-up)
Ensure that the `sourceEventType` parameter is actually optional in `PDFViewerApplication.{download, save}` (PR 12248 follow-up)

Without these changes, clicking on the "Open With Different Viewer"-button on the Firefox fallback bar won't actually do anything and the following is printed in the web-console:
```
Uncaught TypeError: (destructured parameter) is undefined
    download resource://pdf.js/web/viewer.js:956
    response resource://pdf.js/web/viewer.js:1054
    listener resource://pdf.js/web/viewer.js:11891
viewer.js:956:1
```

Furthermore, this patch also fixes `PDFViewerApplication.fallback` to pass in an explicit `sourceEventType` when triggering downloading. While this, on its own, would obviously have been sufficient to fix the bug described above, it seems wrong to outright break backwards compatibility of any older `PDFViewerApplication.download` calls.

timvandermeij/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,xavier114fch/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,xavier114fch/pdf.js,macroplant/pdf.js,caremerge/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,caremerge/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js