Add OpacityPiecewiseFunction support to vtkMitkLevelWindowFilter
Fix ScalarOpacityFunction part of TransferFunctionProperty ignored
Merge branch 'bug-15899-ImageVtkMapper2DIgnoresOpacity' into bug-15899-ImageVtkMapper2DIgnoresOpacityIntegration
added a rendering test for ImageVtkMapper2DOpacityTransferFunction
added the mitkImageVtkMapper2DOpacityTransferFunctionTest
Merge branch 'master' into bug-15899-ImageVtkMapper2DIgnoresOpacityIntegration

Conflicts:
	Core/Code/Rendering/vtkMitkLevelWindowFilter.cpp
	Core/Code/Rendering/vtkMitkLevelWindowFilter.h
	Core/Code/Testing/CMakeLists.txt
Added scalar opacity transfer function test data.
Merge branch 'bug-15899-ImageVtkMapper2DIgnoresOpacityIntegration'