R1:fd1e0bc4d302

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