Correct DualDepthPeeling for RGBA targets.
Correct DualDepthPeeling for RGBA targets.

The final blending pass had assumed that the target buffer was RGB,
not RGBA.

The new implementation correctly computes the final alpha values of
the blended opaque and translucent layers.

SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix