implemented context sensitive value invalidation in NodeInput

NodeInput uses keyframe information to intelligently determine exactly which
time ranges must be re-cached.

app/node/input.h

@@ -189,6 +189,11 @@ private:
    */
   void insert_keyframe_internal(NodeKeyframePtr key);
 
+  /**
+   * @brief Return whether the standard value should be used over keyframe data
+   */
+  bool is_using_standard_value() const;
+
   /**
    * @brief Internal list of accepted data types
    *