Correctly handle results of morphological operations

Summary: In the morphological operations, the (ITK image) result was always overwriting the original (MITK) image. This caused the image to lose previous information that is not contained in an ITK image. It was only noticeable now because since the last release the Segmentation Interpolation automatically tries to update itself based on the new image, but causes an error while cloning it to a mitk::LabelSetImage because information is missing. Fixes T29608

Test Plan: Load an image, create a segmentation & draw a bit, then use the morphological operations in the Segmentation Utilities view. Should work without crashes.

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T29608

Differential Revision: https://phabricator.mitk.org/D882