Add PIGZ Compression to a global/image item.  If the image item isn't specified it will default to that of the default global.  If the image item is specified, it will default to that of the image compression rating.

git-svn-id: https://svn.code.sf.net/p/freeghost/code/trunk@2696 71f96598-fa45-0410-b640-bcd6f8691b32