Fix vtkImageSliceRepresentation's mtime.

Fixes issue after commit 07671104. The data object now needs to be modified in
RequestData() for the delivery code to detect that it was indeed updated. This
was causing pvcs.Slice and pvcrs.Slice tests to fail. This fixes it.

Change-Id: I2040045cd88b94b39509551cc5f30ae1a582245c

ParaViewCore/ClientServerCore/Rendering/vtkImageSliceRepresentation.cxx

@@ -166,8 +166,17 @@ int vtkImageSliceRepresentation::RequestData(vtkInformation* request,
   if (inputVector[0]->GetNumberOfInformationObjects()==1)
     {
     this->UpdateSliceData(inputVector); 
-    this->CacheKeeper->Update();
     }
+  else
+    {
+    // This happens on the client in client-server mode. We simply mark the data
+    // dirty so that the "delivery" code can fetch new data.
+    if (!this->GetUsingCacheForUpdate())
+      {
+      this->SliceData->Modified();
+      }
+    }
+  this->CacheKeeper->Update();
 
   return this->Superclass::RequestData(request, inputVector, outputVector);
 }