Review of vtkCocoaRenderWindow SetSize and SetPosition

- fixed error in vtkCocoaRenderWindow::SetPosition() where it was unconditionally setting the NSWindow position to 0,0
- eliminated recursion/reentry into viewFrameDidChange: by adding checks in SetSize() and SetPosition()
- slightly tweaked vtkCocoaRenderWindow::GetSize implementation to return 0,0 if somehow there is no NSView, which is an unlikely (impossible?) edge case
- rearranged some tests so the faster ones are first
- improved some comments