When an image is too wide in Pholio, scale it down

Summary:
Currently, if an image is too wide for the viewport, we freak out. Instead, scale it down.

This means we must also scale down all the rectangles on it, which is why this is tricky. However, all the draw/load separation has made it reasonably straightforward.

We'll possibly need to add some kind of "view full size" thing. I'm planning to add an element which shows "85%" or whatever if it's currently scaled.

Test Plan:
Before:

{F33607}

After:

{F33608}

Reviewers: chad, ljalonen

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5088