vtkScatterPlotMatrix: persist plots across animation frames

- The 2D plots would unnecessarily invoke draw commands during each animation frame even though no data points were modified.
- With this commit, only the 3D chart gets redrawn for animations.
- Moved 3D chart into a separate context actor with it's own renderer.
- The main renderer is asked not to erase contents during the start of animation.
- During animation, the 3D renderer is carefully placed such that it's viewport takes up the previous 2D chart's space.
- After animation, the 3D renderer is removed from the window and the main renderer is asked to erase contents before drawing.