Fix streaming in vtkImageMarchingCubes
This filter wasn't streaming because the input's whole extent was updated before vtkImageMarchingCubes::RequestData() was called. In order for streaming to work properly, the input data must not be updated before RequestData() updates it. To state this in another way, setting a memory limit for the input has no effect if the whole input is filled before vtkImageMarchingCubes even executes. To properly test this filter, the test was modified to use a streaming reader (vtkImageReader2 instead of vtkVolume16Reader), and the InputMemoryLimit was reduced.
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