make nodes more const friendly and prepare their functions for stateless work

Since the nodes won't be holding any rendering data themselves in this system,
we may as well enforce some level of non-write access by setting all the
functions to const. They were already const-friendly, they just weren't
labelled as such.

app/node/output/viewer/viewer.cpp

@@ -35,27 +35,27 @@ ViewerOutput::ViewerOutput()
   AddParameter(length_input_);
 }
 
-Node *ViewerOutput::copy()
+Node *ViewerOutput::copy() const
 {
   return new ViewerOutput();
 }
 
-QString ViewerOutput::Name()
+QString ViewerOutput::Name() const
 {
   return tr("Viewer");
 }
 
-QString ViewerOutput::id()
+QString ViewerOutput::id() const
 {
   return "org.olivevideoeditor.Olive.vieweroutput";
 }
 
-QString ViewerOutput::Category()
+QString ViewerOutput::Category() const
 {
   return tr("Output");
 }
 
-QString ViewerOutput::Description()
+QString ViewerOutput::Description() const
 {
   return tr("Interface between a Viewer panel and the node system.");
 }
@@ -115,7 +115,8 @@ void ViewerOutput::set_audio_params(const AudioParams &audio)
 
 rational ViewerOutput::Length()
 {
-  return length_input_->get_realtime_value_of_connected_output().value<rational>();
+  // FIXME: This is pretty messy, there's probably a better way...
+  return length_input_->get_connected_node()->Value(NodeValueDatabase()).Get(NodeParam::kRational).value<rational>();
 }
 
 void ViewerOutput::DependentEdgeChanged(NodeInput *from)