Fix geometry sizing such that result is fit within specified maximum dimensions

According to [ImageMagick docs](http://www.imagemagick.org/script/command-line-processing.php#geometry) specifying a geometry in the form `300x200` states that the maximum dimensions of the image are provided and the image should be resized to **fit within** these while maintaining aspect ratio.

Prior to this PR when using the Imgry::Geometry calculations, the resulting image dimensions would sometimes treat the requested dimension as maximum output size and sometimes treat the dimension as minimum output size, depending on aspect ratio of the original and the request.  This PR makes the calculated geometry consistent with image magick.