Add a particular type of resource to the page.
Pages may contain default resources usable by the page content
stream. The most common resource types are "Font", "XObject" and
"ColorSpace". For further details see the PDF Specification.
This method allows you to add new resources to the page. You may
supply your own name for the resource but if the name is already in
use, it may need to be modified. For this reason the function
returns the value which was actually used for the addition.