This 10M triangle scene stresses depth peeling implementations to see
how well they perform.

Results (nVidia Quadro 2000M, Release build):

vtkDepthPeelingPass: 1.2 FPS (0.77s first frame, 0.83s subsequent)
vtkDualDepthPeelingPass: 2.5 FPS (0.56s first frame, 0.40s subsequent)

We see a roughly 50% improvement with the new dual algorithm, since
the dual algorithm peels 2 layers per geometry pass.

For reference, simple alpha blending results in 49 FPS, 0.18s first frame,
0.020s subsequent.