Formulas are not showing up in PDF documents if document is not public

Like for many macros that generate images, the PDF export will only be able to export the image if the wiki page is public.

Some additional code is needed to save the image locally in case of PDF export in order for the PDF export to not be dependent on being able to access a resources at a specific URL served by XWiki.

How to reproduce:

1. Install standard XWiki
2. Add formula

   {{formula}}
   x / 3
   {{/formula}}
   

3. Protect page for admin
4. Export as PDF

The equation won't be present