Allow displaying LUT in PCLVisualizer when adding a vtkPolyData

[VictorLamoine]

Colors are nice (#1143) but the LUT cannot be displayed, when U is pressed in the visualizer:

Warning: In /build/buildd/vtk-5.8.0/Rendering/vtkScalarBarActor.cxx, line 270
vtkScalarBarActor (0x840a9a0): Need a mapper to render a scalar bar

A problem arise if the user has multiple vtkPolyData in the viewer and the LUTs are not the same

• Minimum/maximum scalar values are unlikely to be the same
• The user might have chosen 2 different LUTs for the vtkPolyData

https://github.com/PointCloudLibrary/pcl/blob/master/visualization/src/interactor_style.cpp#L1057

[taketwo]

Well, if there are two different LUTs, then it's not much we can do about it. But if there is only one (which I expect to be the case most often), then it would be nice to have a color bar for it.

[VictorLamoine]

Ok, my problem is that there is not shape_actor_map_ (see https://github.com/PointCloudLibrary/pcl/blob/master/visualization/include/pcl/visualization/interactor_style.h#L261) in the interactor_style.h

I don't know how to search for existing shape actors in the renderer.

EDIT: Got it: PCLVisualizer initializes PCLVisualizerInteractorStyle->actors_ with cloud_actor_map (https://github.com/PointCloudLibrary/pcl/blob/master/visualization/src/pcl_visualizer.cpp#L167). The equivalent for shapes is missing