fix bug when using timing.isFixed, closes #44

Author: liabru

src/core/Runner.js

@@ -42,7 +42,7 @@ var Runner = {};
         (function render(time){
             var timing = engine.timing,
                 delta,
-                correction;
+                correction = 1;
 
             timing.frameRequestId = _requestAnimationFrame(render);
 
@@ -123,4 +123,4 @@ var Runner = {};
         _cancelAnimationFrame(engine.timing.frameRequestId);
     };
 
-})();
+})();