Lighting does't work in 07-3D-03-PyramidLighting 

Lighting doesn't work with this sample.
It seems like ofBackgroundGradient is related.

I tried to add some code inside ofApp::draw(){} and looks it works.

```
ofBackgroundGradient( ofColor( 255 ), ofColor( 128 ) );
```

 ->

```
ofDisableLighting();
ofBackgroundGradient( ofColor( 255 ), ofColor( 128 ) );
ofEnableLighting();
```
