Unexpected behavior when combining filters

[mrobinson]

I have observed that opacity filters do not combine as I would expect. For instance the two rectangles in the following wrench output are a different color:

root:
  items:
    - type: stacking-context
      bounds: [10, 10, 500, 500]
      filters: [opacity(0.50), opacity(0.50)]
      items:
        - type: rect
          bounds: [10, 10, 100, 100]
          color: green
    - type: stacking-context
      bounds: [110, 10, 500, 500]
      filters: [opacity(0.25)]
      items:
        - type: rect
          bounds: [10, 10, 100, 100]
          color: green

[mrobinson]

@glennw Perhaps this is an issue with premultiplied alpha?