added clone functions for nodes

Since nodes are are extremely derivative types, we use a clone/copy function
to produce new types of the derived class easily.

app/node/color/opacity/opacity.cpp

@@ -37,6 +37,11 @@ OpacityNode::OpacityNode()
   AddParameter(texture_output_);
 }
 
+Node *OpacityNode::copy()
+{
+  return new OpacityNode();
+}
+
 QString OpacityNode::Name()
 {
   return tr("Opacity");

app/node/color/opacity/opacity.h

@@ -29,6 +29,8 @@ class OpacityNode : public Node
 public:
   OpacityNode();
 
+  virtual Node* copy() override;
+
   virtual QString Name() override;
   virtual QString Category() override;
   virtual QString Description() override;