Changed public API of vtkGPUInfo to return vtkTypeUInt64 for memory sizes.
It previously used vtkIdType which could be 32 bit even on 64 bit systems. Fixes overflow in the case of 32 bit vtkIdType and GPUs with more than 2 GiB of memory. Also changed "MB" to "MiB" in test output, since it's base 2. Change-Id: I23ba36647596cc49e3a65fab5784e18c021070e1