Composing AlphaCompositedImages bug

Hi,

when composing two AlphaCompositedImages that both have pixels with
alpha values > 0 and < 1.0, the alpha value is not computed, so the
composed image will have the alpha value of the target image.

proposed fix:

AlphaCompositedImage>>directCompositeOnto:box:
   ...
   alpha = 0
     ifFalse: [
       " patch >>> "
       backAlpha := targetBits at: index.
       backAlpha ~~ alpha
          ifTrue: [
             targetBits at: index put:
                 ((alpha + backAlpha) * 255 -
                    ((alpha * backAlpha ))) // 255.
          ].
       " <<< patch "
       index + 1 to: index + 3 do: [
          ....
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