Recent commit 122b241e (Define vtkIOStream(Pos|Off) as portable
std::stream(pos|off), 2012-05-04) added these for portability but the
standard types are available everywhere now that we support only
standard streams.  We can use std::stream(pos|off) directly instead.

Recent commit 6d2b9838 (vtk(Input|Output)Stream: Use appropriate types
in API, 2012-05-04) exposed std::streamoff (through vtkIOStreamOff) in
the public interface of vtkInputStream and vtkBase64InputStream.  In
order to present a consistent public interface use vtkTypeInt64 instead
because std::streamoff varies on some platforms depending on large file
support preprocessor definitions.

Change-Id: I8d2003a353e6cb7f010be631daf4e234eeec678b