Whether width or height or both are used as the screen size should be configurable

From issue: http://drupal.org/node/1467216

Currently, the screen size used to determine which style to use is the result of max(width, height). This is so it works equally well with portrait or landscape images.

Unfortunately this general case does not work well if you're using exclusively portrait or landscape images.

How the detection works should be configurable between:

• Maximum of Width and Height
• Minimum of Width and Height
• Width Only
• Height Only