In Port Default
Macro to replace unconnected in-port with default value
Guaranteed to output a single event when:
* In port is connected (outputs connected value as init-derived event)
* In port is disconnected (outputs default as init-derived event)
* In port has event (outputs regular event)
* Init (outputs either connected port value or default as init-derived event)
Based on https://www.native-instruments.com/en/reaktor-community/reaktor-user-library/entry/show/13977/ and much hair pulling when trying to correctly handle unconnected defaults.