Fix: 'opacity' workaround in ImageViewer (PCLVisualizer)

Hi,
during testing the ImageViewer with 'PCLContextItem' (vtkContextItem) I realized that different values of 'opacity' settings had no effekt on screen. I found out that the vtk pens and brushes have to be set with rgba-colors on client side in the 'paint' functions. After several tests I can say that everything works fine. 

The 'opacity' member in 'struct PCLContextItem' is needless, because its almost a member of 'vtkContextItem'.