Previously, the `TransferFunction` worklet required you to call
`PrepareForExecution` on the `ColorTable` and give that to the
constructor. Bad things can happen if the `ColorTable` used in the
constructor does not match the device the worklet actually gets invoked
on. Change this to make the `ColorTable` a worklet argument of type
`ExecObj`. The dispatcher will automatically call
`ColorTable::PrepareForInput` and guarantee that the devices match.