Issue #4692 - Move responsibility for engine reference from tasks to scheduler

src/Scheduler.h

Index: src/Scheduler.h
===================================================================
--- a/src/Scheduler.h
+++ b/src/Scheduler.h
@@ -66,7 +66,24 @@
   }
 }
 
-/*
+/**
+ * Interface class for scheduled tasks.
+ */
+struct TaskFunctionInterface
+{
+  /** 
+   * The main function of the task,
+   *   the moral equivalent to the main function of the thread.
+   */
+  virtual void operator()() = 0;
+  /**
+   * Request that the task end itself early, without needing to complete.
+   * Reserved for future use.
+   */
+  virtual void Interrupt() {};
+};
+
+/**
  * Execute a task immediately in detached thread that's used only for this task.
  *
  * The present version is nothing more than a rewrite of the legacy behavior,