more coherent render threading for now

Once again, conceptually this system should work, however it does not seem to
be the most efficient and it wouldn't surprise me if the multithreading was
eventually upgraded to an even more coherent system one day. However for
"core principles" this should be fairly decent.

app/node/node.h

@@ -129,6 +129,11 @@ public:
    */
   virtual QString Code(NodeOutput* output);
 
+  /**
+   * @brief Returns the parameter with the specified ID (or nullptr if it doesn't exist)
+   */
+  NodeParam* GetParameterWithID(const QString& id);
+
   /**
    * @brief Returns whether this Node outputs data to the Node `n` in any way
    */
@@ -199,6 +204,7 @@ public:
    */
   void LockProcessing();
   void UnlockProcessing();
+  bool IsProcessingLocked();
 
   /**
    * @brief Copies inputs from from Node to another including connections