Fix: reduce framerate overhead on train ticks

Tick() is a noop for all but front-engine / crashed vehicles. Starting a framerate is rather cheap, but not free, and introduces a lot of overhead for such close loops.

src/train_cmd.cpp

@@ -3910,11 +3910,11 @@ Money Train::GetRunningCost() const
  */
 bool Train::Tick()
 {
-	PerformanceAccumulator framerate(PFE_GL_TRAINS);
-
 	this->tick_counter++;
 
 	if (this->IsFrontEngine()) {
+		PerformanceAccumulator framerate(PFE_GL_TRAINS);
+
 		if (!(this->vehstatus & VS_STOPPED) || this->cur_speed > 0) this->running_ticks++;
 
 		this->current_order_time++;