Add outline glow render pass

Outline glow pass draws a glowing outline of a scene. Combined with layered renderers this can be used to highlight parts of a scene. Outline glow uses render passes to first draw the scene, apply gaussian blur and then draw the blurred image only where no pixels were drawn in the original scene. The result is an outline with a hard edge on the inside and a smooth edge on the outside.

In order to make the layered rendering approach work, I had to make one alteration to an already existing vtk class. A vtkImageProcessingPass will now clear its render target before rendering to it. Without this, the render target would be filled with whatever data is in the graphics memory of the render target.

I could not observe any impact of this change on other render passes derived from vtkImageProcessingPass. But then again, I am not familiar with all the intricacies of vtk's rendering.