Add missing image functions to PHP extension functions list

Summary:
Copied from PHP Manual.

Also fix missing cache key for implicitly linted file.

Test Plan:
  phabricator/ $ arc lint src/applications/files/PhabricatorImageTransformer.php # without GD2

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Differential Revision: https://secure.phabricator.com/D5457