Implement error-checked "preview" transforms
Summary: Ref T7707. These transforms have a single maximum dimension instead of fixed X and Y dimensions.
Test Plan: Transformed a bunch of files with different sizes/aspect ratios, got sensible results.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7707
Differential Revision: https://secure.phabricator.com/D12812