renderer: share render backend for auto-cache tasks

Since auto-cache events start and stop fairly frequently (and are somewhat
heavy to create/destroy), we can save a lot of cycles over time by sharing
the same backend between them all.

app/node/node.cpp

@@ -537,6 +537,25 @@ bool Node::OutputsTo(const QString &id, bool recursively) const
   return false;
 }
 
+bool Node::OutputsTo(NodeInput *input, bool recursively) const
+{
+  QList<NodeOutput*> outputs = GetOutputs();
+
+  foreach (NodeOutput* output, outputs) {
+    foreach (NodeEdgePtr edge, output->edges()) {
+      NodeInput* connected = edge->input();
+
+      if (connected == input) {
+        return true;
+      } else if (recursively && connected->parentNode()->OutputsTo(input, recursively)) {
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
 bool Node::InputsFrom(Node *n, bool recursively) const
 {
   QList<NodeInput*> inputs = GetInputsIncludingArrays();

app/node/node.h

@@ -220,6 +220,11 @@ public:
    */
   bool OutputsTo(const QString& id, bool recursively) const;
 
+  /**
+   * @brief Same as OutputsTo(Node*), but for a specific node input rather than just a node.
+   */
+  bool OutputsTo(NodeInput* input, bool recursively) const;
+
   /**
    * @brief Returns whether this node ever receives an input from a particular node instance
    */