Add option to independently set thickness of cell edges and lines

You can now set the thickness of cell edges independently without altering the thickness of lines. This feature is useful to emphasize either edges or lines in a scene with different widths.

The vtkProperty class now offers two new options to deal with edges:

1. vtkProperty::SetEdgeWidth(float) controls the thickness of cell edges.

2. vtkProperty::SetUseLineWidthForEdgeThickness(bool) specifies whether the thickness of edges is equal the value of LineWidth option (default) or the EdgeWidth option (new)

The default value of UseLineWidthForEdgeThickness is true to prevent breaking the behaviour of existing applications. Applications that used LineWidth to control the thickness of edges will continue to work as expected.

As an example, here is a single polydata that has both polygonal faces and polylines. Notice how the edges are wider than the lines. You can achieve this by configuring EdgeWidth and turning off UseLineWidthForEdgeThickness on the actors’ vtkProperty instance.