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.