hardware protocol v2: answer a subscription with current state

The snapshot sent right after SUBSCRIBE was taken from the previous
frame, which on the first frame of a session meant every value came back
as UNAVAILABLE. The link now publishes the state of the current frame
before the received frames are processed.

hardware/hardware_manager.cpp

@@ -202,6 +202,9 @@ void hardware_link::update( double const Deltatime, state_snapshot const &Snapsh
 	m_lastframeerrors = counters;
 	m_session.add_frame_errors( delta );
 
+	// the session answers a subscription with an immediate snapshot, so it needs the current state first
+	m_session.set_snapshot( Snapshot );
+
 	for( auto const &packet : packets )
 	{
 		m_session.handle_packet( packet );

hardware/protocol/protocol_session.h

@@ -49,6 +49,8 @@ public:
 	void reset();
 	// consumes one decoded frame
 	void handle_packet( decoded_packet const &Packet );
+	// publishes the state of the current simulation frame, before the received frames are processed
+	void set_snapshot( state_snapshot const &Snapshot ) { m_snapshot = Snapshot; }
 	// periodic work: heartbeats, subscription updates, timeouts
 	void update( double const Deltatime, state_snapshot const &Snapshot );
 	// copies the current condition into the debug report