sweeping node graph changes for new rendering pipeline

The new rendering pipeline strives to simplify the nodes themselves as much
as possible and move much of the logic to an external rendering engine. This
change removes all of the responsibilities that no longer belong to the
nodes themselves and will soon be folded into the renderer.

app/node/output/viewer/viewer.cpp

@@ -104,14 +104,5 @@ void ViewerOutput::set_audio_params(const AudioParams &audio)
 
 rational ViewerOutput::Length()
 {
-  return length_input_->get_value(0).value<rational>();
-}
-
-QVariant ViewerOutput::Value(NodeOutput *output, const rational &in, const rational &out)
-{
-  Q_UNUSED(output)
-  Q_UNUSED(in)
-  Q_UNUSED(out)
-
-  return 0;
+  return length_input_->get_realtime_value_of_connected_output().value<rational>();
 }