Fix weird PolygonGeometry perPositionHeight edge case

[hpinkos]

Fixes crash when perPositionHeight: true, extrudedHeight is defined, and vertexFormat: VertexFormat.POSITION_ONLY.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var positions = Cesium.Cartesian3.fromDegreesArrayHeights([
    -108.0, 25.0, 100000,
    -100.0, 25.0, 100000,
    -100.0, 30.0, 100000,
    -108.0, 30.0, 300000
]);
var orangePolygonInstance = new Cesium.GeometryInstance({
    geometry : Cesium.PolygonGeometry.fromPositions({
        positions : positions,
        extrudedHeight: 0,
        vertexFormat : Cesium.PerInstanceColorAppearance.FLAT_VERTEX_FORMAT,
        perPositionHeight : true
    }),
    attributes: {
        color: Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.ORANGE)
    }
});

scene.primitives.add(new Cesium.Primitive({
    geometryInstances : [orangePolygonInstance],
    appearance : new Cesium.PerInstanceColorAppearance({
        flat: true,
        closed : true,
        translucent : false
    })
}));

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[bagnell]

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