img test: skip SVG tests if unsupported by Image::Magick
img: raise an error if we cannot find the image's size

This happens for PDFs without ghostscript installed, for instance.
Add package dependencies for scalable img support

- suggest ghostscript (required for PDF-to-PNG thumbnailing)
  and libmagickcore-extra (required for SVG-to-PNG thumbnailing)
- build-depend on ghostscript so the test for scalable images can be run
Add myself to Uploaders and release to Debian