Fix Visual Studio 2017 warnings (errors) in RegionGrowingTool
Summary:
Visual Studio 2017 insists of this being an implicit narrowing cast even
though it is a widening cast.
Test Plan: @kalali already confirmed that the fix compiles in Visual Studio 2017.
Reviewers: kalali, O1 MITK Reviewer Group I
Reviewed By: kalali, O1 MITK Reviewer Group I
Subscribers: kalali
Maniphest Tasks: T27668
Differential Revision: https://phabricator.mitk.org/D385