Add a particular type of resource to the Form XObject.
Form XObjects 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
This method allows you to add new resources to the Form XObject.
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