XPS: fix multipiece image loading

A Slackware user reported that Okular is unable to load some XPS files: https://www.linuxquestions.org/questions/slackware-14/okular-doesn%27t-display-xps-files-4175706793/ The file itself gets loaded and rendered as an empty, white page. Turns out these files only contains an image, and this image is saved in the xps archive as a directory containing multiple pieces, the first one being the image itself and the second one being empty. Of course this is quite a weird way to save an image, but it seems that it still is valid and other viewers are able to load it correctly. This change fixes the loading of multipiece images.