Added back in rendering of selected points

This was lost when the point plotting was moved to a dedicated class out
of the vtkChartXYZ class. Restore this behavior, which is used in the
scatter plot matrix animations for example.

Change-Id: I1f69b4d45aef25566f8e8cd644f72245506b975e
Merge topic 'chart-xyz-selection' into master

da0126c0 Added back in rendering of selected points