improve image hinting for 220 preview
Summary: break out the calculation of dimensions as a static method and use it
Test Plan: made a conpherence with many images and noted i auto-scrolled to the bottom correctly
Reviewers: chad, epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2399
Differential Revision: https://secure.phabricator.com/D4733