[Monitor] center the monitor when not attached to a transform (#385)

unity-environment/Assets/ML-Agents/Scripts/Monitor.cs

             float paddingwidth = 10 * widthScaler;
 
             float scale = 1f;
-            var origin = new Vector3(0, Screen.height);
+            var origin = new Vector3(Screen.width / 2 - keyPixelWidth, Screen.height);
             if (!(target == canvas.transform))
             {
                 Vector3 cam2obj = target.position - Camera.main.transform.position;