Visible GroundPrimitive seam at IDL and Prime Meridian


For `GroundPrimitives`, when the geometry is sufficiently large, there's a weird artifact at `longitude = 0` where we split into east and west hemispheres in `GeometryPipeline.splitLongitudeTriangles`.
(The same thing will happen across the IDL if you flip the east and west coordinates in the example below)

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');
var rectangle = Cesium.Rectangle.fromDegrees(-125.0, -30.0, 125.0, 30.0);

var primitive = viewer.scene.primitives.add(new Cesium.GroundPrimitive({
geometryInstances : [new Cesium.GeometryInstance({
geometry : new Cesium.RectangleGeometry({
rectangle : rectangle,
vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
}),
attributes: {
color: Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 0.0, 0.0, 0.8))
}
})],
appearance : new Cesium.PerInstanceColorAppearance({
closed : true
})
}));
```