Fixed handling of RGB and RGBA images
Summary: Some minor code changes to fix an itk bug, add tests
Test Plan: Added automatic test
Reviewers: O1 MITK Core, kislinsk
Reviewed By: O1 MITK Core, kislinsk
Subscribers: kislinsk
Maniphest Tasks: T20223
Differential Revision: https://phabricator.mitk.org/D21