[VTK-5280] Resolve IMAGE_REF for multi-value props as well.
[VTK-5280] Resolve IMAGE_REF for multi-value props as well.

However, one of the utility methods will still fail, since it returns a single
String value, which is hard to map multiple values onto.

vtkio/vtk,vtkio/vtk,vtkio/vtk,vtkio/vtk,vtkio/vtk,vtkio/vtk,vtkio/vtk,vtkio/vtk