Fix drag-inline behavior with pholio stage background
Summary: Currently, if you click and drag in the checkered background, we'll create a 1px-wide or 1px-tall inline comment. Don't start inlines unless the user clicks on the image itself.
Test Plan: Clicked on and off the image.
Reviewers: chad
Reviewed By: chad
CC: aran
Differential Revision: https://secure.phabricator.com/D5227