When the engine is restarted, ports are re-established,
and previously queued port-connections need to be cleared.
This caused a bug:
* when the engine is stopped all ports are disconnected.
_port_connection_queue contains all disconnections
* engine is stopped so _port_connection_queue is not processed
* engine-restart re-etablishes ports and appends those connections
to _port_connection_queue
* process-callback processes the list in **reverse** order
```
while (!_port_connection_queue.empty ()) {
_port_connection_queue.pop_back ();
}
```
* ARDOUR::PortManager::connect_callback() is first called
with connected() and the disconnected()
* All ports are assumed to not be connected
Port::_externally_connected == 0 for all ports
Result:
* vari-speed playback resampling does not work (only external
I/O is reampled), split cycles processing (looping) fails
since AudioPort::get_audio_buffer() does not apply the
_global_port_buffer_offset
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