mason/{.perl-lib/TrivGal,dhandler}: Make an image object.

mason/{.perl-lib/TrivGal,dhandler}: Make an image object.

The objective here is to separate image loading from scaling, so that we
can make several scaled output images without having to load and decode
the original image each time.

The `.thumbnail' module now takes an image object rather than a
pathname.  There's no functional change.