Ground primitive clipping issue

[mramato]

@bagnell here's the bug we were talking about yesterday, not sure if you have a better test case (this one requires @tfili's entity-terrain branch.

Run the below code in Sandcastle and then hit the "Bug" button. Right-click zoom in and out of the rectangle (without rotating) and you will see obvious visual artifacts as the ground primitive seemingly changes size/shape as you zoom. I believe @bagnell said this is a multi-frustum related issue.

var viewer = new Cesium.Viewer('cesiumContainer', {
    baseLayerPicker: false,
    terrainProvider: new Cesium.CesiumTerrainProvider({
        url: '//assets.agi.com/stk-terrain/world'
    })
});


var rect = Cesium.Rectangle.fromDegrees(-85.0, 35.99, -84.99, 36.0);
Sandcastle.addToolbarButton('Bug', function () {
    viewer.entities.add({
        rectangle: {
            coordinates: rect
        }
    });

    viewer.camera.setView({destination: rect});
});

[bagnell]

Updated code that does not require the entity-terrain branch:

    var viewer = new Cesium.Viewer('cesiumContainer', {
        baseLayerPicker: false,
        terrainProvider: new Cesium.CesiumTerrainProvider({
            url: '//assets.agi.com/stk-terrain/world'
        })
    });

    var rect = Cesium.Rectangle.fromDegrees(-85.0, 35.99, -84.99, 36.0);
    viewer.camera.setView({destination: rect});

    Sandcastle.addToolbarButton('Bug', function () {
        viewer.scene.groundPrimitives.add(new Cesium.GroundPrimitive({
            geometryInstances : new Cesium.GeometryInstance({
                geometry : new Cesium.RectangleGeometry({
                    rectangle : rect
                }),
                attributes: {
                    color: Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 1.0, 0.0, 0.5))
                }
            }),
            asynchronous : false
        }));
    });

[mramato]

Thanks @bagnell