Fix the rim lighting in GridMaterial to accept backface normals.

This is what it looked like before.  The dividing line between good & bad is actually a separate issue with `faceForward` that will be addressed in a separate pull.  This pull just fixes the bad side to look like the good side.

![badbackfacenormals](https://f.cloud.github.com/assets/1235080/2263132/0ed978c8-9e62-11e3-82b1-52e8aca2a788.png)