Fix the temporary fix for planar figures in MxN

Summary: The fix introduced in D801 was a bit too harsh and filtered not only for MxN windows, but everything that was not a Std window, which caused some tests with custom windows to fail. Now the MxN windows include a "mxn." prefix and are specifically filtered for in the temporary fix.

Test Plan: See D801. Also, tests should no longer fail (specifically mitkViewportRenderingTest)

Reviewers: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kislinsk

Reviewed By: O1 MITK Reviewer Group I, O3 MITK Reviewer Group II, kislinsk

Differential Revision: https://phabricator.mitk.org/D807