Disable the server-side image resizing while exporting to PDF

The images displayed by a wiki page are normally included in the PDF export in their full size as they are downloaded by the browser generating the PDF. The problem is that the XWiki Rendering performs server-side image resizing when image dimensions are specified in wiki syntax, e.g.:

[[image:test.png||width="200px"]]

In this case the browser downloads a resized image. This makes sense for web page rendering, because it can improve significantly the initial page loading time, but it doesn't make sense for PDF export where we want to have the original image. This is even more important when you take into account that PDFs are often zoomed (unlike web pages), so scaling the images should give good results (if the original image is large enough).

We should disable the server-side image resizing while performing the PDF export.