Added a new flag to vtkImageBlend called CompoundAlpha

The flag CompoundAlpha affects the blending computation in the compound
blending mode of vtkImageBlend. When the flag is off, vtkImageBlend
behaves as it used to and the output alpha values are the sum of the
product of individual opacity and alpha values in the inputs i.e.
output alpha A = (O1A1 + O2A2)

When the flag is on, the blend computation is also applied to the alpha
component of the inputs i.e.
output alpha A = (O1A1A1 + O2A2A2) / (O1A1 + O2A2)