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