mason/.perl-lib/TrivGal.pm (Image): Use `Image::Size' to find image size.

Using `Image::Imlib2' means that we don't learn the size without fully
decoding the image and using up lots of memory.  The `Image::Size'
package will determine the image size by reading it out of the file
without decoding, so it's much faster.

Of course, that doesn't matter much yet, because we're only using it
/after/ we decode the image.  Hmm...