Add AIStateMachine::yield_if_not

This can be used to switch to a specific engine.
If the state machine is not running in the passed engine,
then it performs a yield to that engine, otherwise it keeps
running. Hence, putting this at the top of a state
guarantees that it runs in that engine.

For example:

    case FrontEnd_done:
    {
      // Unlock must be called by the same thread that locked it.
      if (yield_if_not(&gStateMachineThreadEngine))
      {
        break;
      }
      // Here we are running in gStateMachineThreadEngine.
    ...

indra/aistatemachine/aistatemachine.cpp

@@ -1173,6 +1173,16 @@ void AIStateMachine::yield(AIEngine* engine)
   mYieldEngine = engine;
 }
 
+bool AIStateMachine::yield_if_not(AIEngine* engine)
+{
+  if (engine && multiplex_state_type_rat(mState)->current_engine != engine)
+  {
+	yield(engine);
+	return true;
+  }
+  return false;
+}
+
 void AIStateMachine::yield_frame(unsigned int frames)
 {
   DoutEntering(dc::statemachine(mSMDebug), "AIStateMachine::yield_frame(" << frames << ") [" << (void*)this << "]");

indra/aistatemachine/aistatemachine.h

@@ -237,6 +237,7 @@ class AIStateMachine : public LLThreadSafeRefCount
 	void yield(AIEngine* engine);											// Yield to give CPU to other state machines, but do not go idle. Continue running from engine 'engine'.
 	void yield_frame(unsigned int frames);									// Run from the main-thread engine after at least 'frames' frames have passed.
 	void yield_ms(unsigned int ms);											// Run from the main-thread engine after roughly 'ms' miliseconds have passed.
+	bool yield_if_not(AIEngine* engine);									// Do not really yield, unless the current engine is not 'engine'. Returns true if it switched engine.
 
   public:
 	// This function can be called from multiplex_imp(), but also by a child state machine and