Support imagemagick on new image transform pathway

Summary: Ref T7707. For animated GIFs, use imagemagick if it is available.

Test Plan: Generated small versions of a bunch of different GIFs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7707

Differential Revision: https://secure.phabricator.com/D12813