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.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
    */