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
Merge topic 'vtkGPUInfo-bigger-size' into master

7399f6db Changed public API of vtkGPUInfo to return vtkTypeUInt64 for memory sizes.