Previously, the vtkTooltipItem in a vtkChartHistogram2D would
be painted before the vtkColorLegend, and thus appear behind it.

Ideally, there should be a way to define the order in which
vtkAbstractContextItems at the same level are painted, besides simply
the order in which they are added to the vtkContextScenePrivate.

Change-Id: Idda62c50f3192add1c13e6fcf921303726a37f88