Fixed PlaceObject issue.

[Issue]
  The degree of shape's rotation is wrong.

[Cause]
  When a sprite is replaced without specifying any matrix via PlaceObject, the old matrix is applied.
  And if the identity matrix is specified, we should use it and the old matrix should be overwritten by the identity matrix.
  However, the current implementation does not behave so, instead, treats the identity matrix as null.

[Fix]
  Fixed to distinguish the null matrix and the identity matrix.