VideoStream: create the progressive source stream by node id on the portal path

When the progressive (raster) encoding mode is used over the XDG desktop portal, the source stream was created by casting the node id to a quint64 and calling createStream(objectSerial). The portal supplies a PipeWire node id, not an object serial, so PipeWire looked up a target object by a serial that does not exist: the stream never received any frames (PipeWire error -2, "target not found") and the client only saw a black screen.

Create the stream by object serial only when one was actually provided (the Plasma screencast path); otherwise create it by node id, matching what setPipeWireSource() and the H.264 path already do.