BUG: Fix markups handle scale not updating

Addresses issue introduced in 580e187 which caused markups handle scale to not update.
The original fix was meant to prevent a crash when starting VR with Markups in the scene, however it disabled scale update in all situations.
Fixed by checking GetNeverRendered() instead of GetGenericContext(), before calling GetScreenSize(). Once the window has been rendered at least once, it is safe to call GetScreenSize().

Modules/Loadable/Markups/VTKWidgets/vtkSlicerMarkupsWidgetRepresentation3D.cxx

@@ -1196,7 +1196,6 @@ double vtkSlicerMarkupsWidgetRepresentation3D::GetViewScaleFactorAtPosition(doub
   return viewScaleFactorMmPerPixel;
 }
 
-
 //----------------------------------------------------------------------
 void vtkSlicerMarkupsWidgetRepresentation3D::UpdateViewScaleFactor()
 {
@@ -1206,9 +1205,10 @@ void vtkSlicerMarkupsWidgetRepresentation3D::UpdateViewScaleFactor()
   {
     return;
   }
-  if (!this->Renderer->GetRenderWindow()->GetGenericContext())
+
+  if (this->Renderer->GetRenderWindow()->GetNeverRendered())
   {
-    // Calling GetScreenSize() without a context set will cause a crash.
+    // In VR, calling GetScreenSize() without rendering can cause a crash.
     return;
   }