Move Scatter from Worklet to Dispatcher

Previously, when a Worklet needed a scatter, the scatter object was stored in the Worklet object. That was problematic because that means the Scatter, which is a control object, was shoved into the execution environment.

To prevent that, move the Scatter into the Dispatcher object. The worklet still declares a ScatterType alias, but no longer has a GetScatter method. Instead, the Dispatcher now takes a Scatter object in its constructor. If using the default scatter (ScatterIdentity), the default constructor is used. If using another type of Scatter that requires data to set up its state, then the caller of the worklet needs to provide that to the dispatcher. For convenience, worklets are encouraged to have a MakeScatter method to help construct a proper scatter object.