Strange 2D full map lighting

Probably due to the multiple sun and moon light sources.


Sandcastle code:

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
sceneMode: Cesium.SceneMode.SCENE2D
});
var scene = viewer.scene;

var instances = [];

for (var lon = -175.0; lon < 175.0; lon += 5.0) {
for (var lat = -85.0; lat < 85.0; lat += 5.0) {
instances.push(new Cesium.GeometryInstance({
geometry : new Cesium.RectangleGeometry({
rectangle : Cesium.Rectangle.fromDegrees(lon, lat, lon + 5.0, lat + 5.0),
vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
}),
attributes : {
color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.WHITE)
}
}));
}
}

scene.primitives.add(new Cesium.Primitive({
geometryInstances : instances,
appearance : new Cesium.PerInstanceColorAppearance()
}));
```

Start by running the supplied Sandcastle example with Cesium.Viewer in SCENE2D and inspect how the scene's lighting affects the generated RectangleGeometry primitives. Trace the 2D rendering and lighting path from scene.primitives; done should mean the full map renders with consistent, expected lighting rather than the shown artifact.

Written by the indexing model from the issue text.