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