Allow the Pholio stage to scroll vertically

Summary:
Currently, we limit the image size to make sure that the stage has a constant height and the entire image always fits on screen.

In practice, these don't actually seem like desirable qualities. Instead, focus on giving as many pixels as possible to the image.

Test Plan: See screenshots.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9539