Allow "set center" cropping option for PDF media representations

When cropping images to a potentially non-conforming bounding box, cropping is performed relative to the center of the image and often produces arbitrary and undesirable results. For example, portraits often have the subject's face in the upper third of the image; cropping around the center usually results in omission of upper portion of the subject's face .

A "set center" cropping option is available in the ca_object_representations bundle allowing specification of an alternative center point for images. Setting the alternative center to, for instance, the center of the subject's face ensures that the entire face is retained in the cropped image.

Currently this option is only available for images. It should be extended to support PDF files as well.