Expose 'density' attribute for magickload

Hi John, this change is to expose *magick's String-based `density` attribute.

If you weren't already aware (unlikely), it's used when loading vector formats to control the DPI of the "canvas" resolution prior to rasterisation.

Amongst other things, it will allow sharp and friends to provide improved support for SVG, PDF and Postscript files.

The nip2 test suite passes with this change. I had to modify one of the deprecated source files to get it to compile - not sure how to test this bit still works.