Fix render window initialization

QVTKOpenGLWidget must be used with vtkGenericOpenGLRenderWindow.
This change modifies the initialization of the context view to follow the
typical usage for QVTKOpenGLWidget as presented in the documentation :
https://www.vtk.org/doc/nightly/html/classQVTKOpenGLWidget.html#details

Libs/Visualization/VTK/Widgets/ctkVTKDiscretizableColorTransferWidget.cpp

@@ -57,12 +57,12 @@
 #include <vtkDiscretizableColorTransferFunction.h>
 #include <vtkDoubleArray.h>
 #include <vtkEventQtSlotConnect.h>
+#include <vtkGenericOpenGLRenderWindow.h>
 #include <vtkIntArray.h>
 #include <vtkImageAccumulate.h>
 #include <vtkImageData.h>
 #include <vtkPiecewiseFunction.h>
 #include <vtkRenderer.h>
-#include <vtkRenderWindow.h>
 #include <vtkScalarsToColors.h>
 #include <vtkTable.h>
 
@@ -153,12 +153,18 @@ void ctkVTKDiscretizableColorTransferWidgetPrivate::setupUi(QWidget* widget)
     vtkSmartPointer<vtkScalarsToColorsContextItem>::New();
   this->scalarsToColorsContextView = vtkSmartPointer<vtkContextView> ::New();
 
-  this->scalarsToColorsContextView->GetScene()->AddItem(
-    this->scalarsToColorsContextItem.Get());
+#if CTK_USE_QVTKOPENGLWIDGET
+  vtkSmartPointer<vtkGenericOpenGLRenderWindow> renwin =
+    vtkSmartPointer<vtkGenericOpenGLRenderWindow>::New();
+  this->ScalarsToColorsView->SetRenderWindow(renwin);
+#endif
+
+  this->scalarsToColorsContextView->SetRenderWindow(
+    this->ScalarsToColorsView->GetRenderWindow());
   this->scalarsToColorsContextView->SetInteractor(
     this->ScalarsToColorsView->GetInteractor());
-  this->ScalarsToColorsView->SetRenderWindow(
-    this->scalarsToColorsContextView->GetRenderWindow());
+  this->scalarsToColorsContextView->GetScene()->AddItem(
+    this->scalarsToColorsContextItem.Get());
 
   q->setViewBackgroundColor(QColor(49, 54, 59));