Fixed a bug in point selection for edge split.

We have to clear the cached selection pixel buffer in vtkPVHarwareSelector, otherwise in a continous selection mode, such as continously modifying an edge (split, merge) by selecting edge / vertex points, the selection comes back from paraview will contains the points from previous selection, which will cause these edge operations to fail. BTW, the cleared selection buffer is NOT the selection used for displaying selected geometry (representaiton). For reference: void vtkSMRenderViewProxy::ClearSelectionCache() A bug is file for paraview, http://www.paraview.org/Bug/view.php?id=15814