Support Mask in Invoker

If a worklet has a ScatterType or a MaskType, it is (usually) necessary to pass a constructed Scatter or Mask object to the dispatcher. In the case of a Scatter, it is possible to pass the Scatter object as the second argument to the Invoker (after the worklet as the first argument). However, there is no mechanism to pass the mask object to the dispatcher. Consequently, it is not possible to use the Invoker with a worklet that uses a Mask.