Merge topic 'ctkVTKRenderView' and 'ctkVTKSliceView'

* ctkVTKRenderView:
  ENH: ctkVTKRenderView - Do not render if widget is hidden

* ctkVTKSliceView:
  ENH: ctkVTKSliceView - Do not render if widget is hidden

Libs/Visualization/VTK/Widgets/ctkVTKRenderView.cpp

@@ -263,7 +263,7 @@ void ctkVTKRenderView::forceRender()
   logger.trace(QString("forceRender - RenderEnabled: %1")
                .arg(d->RenderEnabled ? "true" : "false"));
 
-  if (!d->RenderEnabled)
+  if (!d->RenderEnabled || !this->isVisible())
     {
     return;
     }

Libs/Visualization/VTK/Widgets/ctkVTKSliceView.cpp

@@ -120,7 +120,7 @@ void ctkVTKSliceView::scheduleRender()
 void ctkVTKSliceView::forceRender()
 {
   CTK_D(ctkVTKSliceView);
-  if (!d->RenderEnabled)
+  if (!d->RenderEnabled  || !this->isVisible())
     {
     return;
     }