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/node.cpp

@@ -45,13 +45,13 @@ Node::~Node()
   }
 }
 
-QString Node::Category()
+QString Node::Category() const
 {
   // Return an empty category for any nodes that don't use one
   return QString();
 }
 
-QString Node::Description()
+QString Node::Description() const
 {
   // Return an empty string by default
   return QString();
@@ -85,11 +85,11 @@ void Node::AddParameter(NodeParam *param)
   }
 }
 
-QVariant Node::Value(NodeOutput *output)
+NodeValueTable Node::Value(const NodeValueDatabase &value) const
 {
-  Q_UNUSED(output)
+  Q_UNUSED(value)
 
-  return QVariant();
+  return NodeValueTable();
 }
 
 void Node::InvalidateCache(const rational &start_range, const rational &end_range, NodeInput *from)
@@ -99,7 +99,7 @@ void Node::InvalidateCache(const rational &start_range, const rational &end_rang
   SendInvalidateCache(start_range, end_range);
 }
 
-TimeRange Node::InputTimeAdjustment(NodeInput *input, const TimeRange &input_time)
+TimeRange Node::InputTimeAdjustment(NodeInput *input, const TimeRange &input_time) const
 {
   Q_UNUSED(input)
 
@@ -156,26 +156,6 @@ void Node::UnlockUserInput()
   user_input_lock_.unlock();
 }
 
-void Node::LockProcessing()
-{
-  processing_lock_.lock();
-}
-
-void Node::UnlockProcessing()
-{
-  processing_lock_.unlock();
-}
-
-bool Node::IsProcessingLocked()
-{
-  if (processing_lock_.tryLock()) {
-    processing_lock_.unlock();
-    return false;
-  } else {
-    return true;
-  }
-}
-
 void Node::CopyInputs(Node *source, Node *destination, bool include_connections)
 {
   Q_ASSERT(source->id() == destination->id());
@@ -232,7 +212,7 @@ void Node::DuplicateConnectionsBetweenLists(const QList<Node *> &source, const Q
   }
 }
 
-bool Node::CanBeDeleted()
+bool Node::CanBeDeleted() const
 {
   return can_be_deleted_;
 }
@@ -242,12 +222,12 @@ void Node::SetCanBeDeleted(bool s)
   can_be_deleted_ = s;
 }
 
-bool Node::IsBlock()
+bool Node::IsBlock() const
 {
   return false;
 }
 
-bool Node::IsTrack()
+bool Node::IsTrack() const
 {
   return false;
 }
@@ -278,12 +258,12 @@ NodeOutput *Node::LastProcessedOutput()
   return o;
 }
 
-const QList<NodeParam *>& Node::parameters()
+const QList<NodeParam *>& Node::parameters() const
 {
   return params_;
 }
 
-int Node::IndexOfParameter(NodeParam *param)
+int Node::IndexOfParameter(NodeParam *param) const
 {
   return params_.indexOf(param);
 }
@@ -296,7 +276,7 @@ int Node::IndexOfParameter(NodeParam *param)
  * TRUE to recursively traverse each node for a complete dependency graph. FALSE to return only the immediate
  * dependencies.
  */
-void GetDependenciesInternal(Node* n, QList<Node*>& list, bool traverse) {
+void GetDependenciesInternal(const Node* n, QList<Node*>& list, bool traverse) {
   foreach (NodeParam* p, n->parameters()) {
     if (p->type() == NodeParam::kInput) {
       Node* connected = static_cast<NodeInput*>(p)->get_connected_node();
@@ -312,7 +292,7 @@ void GetDependenciesInternal(Node* n, QList<Node*>& list, bool traverse) {
   }
 }
 
-QList<Node *> Node::GetDependencies()
+QList<Node *> Node::GetDependencies() const
 {
   QList<Node *> node_list;
 
@@ -321,7 +301,7 @@ QList<Node *> Node::GetDependencies()
   return node_list;
 }
 
-QList<Node *> Node::GetExclusiveDependencies()
+QList<Node *> Node::GetExclusiveDependencies() const
 {
   QList<Node*> deps = GetDependencies();
 
@@ -356,7 +336,7 @@ QList<Node *> Node::GetExclusiveDependencies()
   return deps;
 }
 
-QList<Node *> Node::GetImmediateDependencies()
+QList<Node *> Node::GetImmediateDependencies() const
 {
   QList<Node *> node_list;
 
@@ -365,14 +345,14 @@ QList<Node *> Node::GetImmediateDependencies()
   return node_list;
 }
 
-QString Node::Code(NodeOutput *output)
+QString Node::Code(NodeOutput *output) const
 {
   Q_UNUSED(output)
 
   return QString();
 }
 
-NodeParam *Node::GetParameterWithID(const QString &id)
+NodeParam *Node::GetParameterWithID(const QString &id) const
 {
   foreach (NodeParam* param, params_) {
     if (param->id() == id) {
@@ -383,7 +363,7 @@ NodeParam *Node::GetParameterWithID(const QString &id)
   return nullptr;
 }
 
-bool Node::OutputsTo(Node *n)
+bool Node::OutputsTo(Node *n) const
 {
   foreach (NodeParam* param, params_) {
     if (param->type() == NodeParam::kOutput) {
@@ -400,22 +380,22 @@ bool Node::OutputsTo(Node *n)
   return false;
 }
 
-bool Node::HasInputs()
+bool Node::HasInputs() const
 {
   return HasParamOfType(NodeParam::kInput, false);
 }
 
-bool Node::HasOutputs()
+bool Node::HasOutputs() const
 {
   return HasParamOfType(NodeParam::kOutput, false);
 }
 
-bool Node::HasConnectedInputs()
+bool Node::HasConnectedInputs() const
 {
   return HasParamOfType(NodeParam::kInput, true);
 }
 
-bool Node::HasConnectedOutputs()
+bool Node::HasConnectedOutputs() const
 {
   return HasParamOfType(NodeParam::kOutput, true);
 }
@@ -432,7 +412,7 @@ QVariant Node::PtrToValue(void *ptr)
   return reinterpret_cast<quintptr>(ptr);
 }
 
-bool Node::HasParamWithID(const QString &id)
+bool Node::HasParamWithID(const QString &id) const
 {
   foreach (NodeParam* p, params_)
   {
@@ -445,7 +425,7 @@ bool Node::HasParamWithID(const QString &id)
   return false;
 }
 
-bool Node::HasParamOfType(NodeParam::Type type, bool must_be_connected)
+bool Node::HasParamOfType(NodeParam::Type type, bool must_be_connected) const
 {
   foreach (NodeParam* p, params_) {
     if (p->type() == type