R1:29e176fe530f

Preserve transparency in thumbnails

Summary: Add some code from a random guy on the Internet.

Test Plan: Upload a PNG file with alpha; check thumbnail.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1646

Differential Revision: https://secure.phabricator.com/D3258