Update render function to use local file when called by dw2pdf

Use dw2pdf pseudo protocol for images as described in dw2pdf docs:
https://www.dokuwiki.org/plugin:dw2pdf#for_developers

It's useful when HTTP-based authentication method is used (like Basic
Auth or certificates) and  mpdf is not able to fetch image over HTTP.
Merge pull request #2 from scibi/dw2pdf

Update render function to use local file when called by dw2pdf