Moved the enum definition of transfer functions to mitkTransferFunction.h and added two transfer functions: TF_MR_MIP and TF_XR_VR.

Added vtkMitkOpenglXRAYVolumeTextureMapper2D that is being used by GPUVolumeMapper3D.

To use XRay rendering (based on example Step3.cpp):

// Set these two properties to true
node->SetProperty("volumerendering", mitk::BoolProperty::New(true));
node->SetProperty("xrayvolumerendering", mitk::BoolProperty::New(true));

// Use this transfer function
mitk::TransferFunction::Pointer tf = mitk::TransferFunction::New();
tf->SetTransferFunctionMode( mitk::TransferFunction::TF_XR_VR );

// Set background to white
renderWindow.GetRenderer()->GetVtkRenderer()->SetBackground( 1, 1, 1 );