Fix rectangles at north and south poles

[hpinkos]

Fixes #7406
Fixes #4760

When the rectangle north is the north pole, it comes to a single point instead of a row of points at the top edge. RectangleGeometry was creating a ton of duplicate points and every other triangle at the north pole had zero area, which was causing things to go weird further up the geometry pipeline (specifically when splitLongitude was trying to use barycentric coordinates to compute attributes).

Similarly for the south pole and the bottom edge.

This change creates a single north pole position and/or south pole position and triangulates accordingly.

For an example, this rectangle crashes in master, but is fixed in this branch:

var viewer = new Cesium.Viewer('cesiumContainer');
viewer.scene.primitives.add(new Cesium.Primitive({
    geometryInstances : new Cesium.GeometryInstance({
        geometry : new Cesium.RectangleGeometry({
            rectangle : Cesium.Rectangle.fromDegrees(-180.0, -90.0, -179.0, 90.0),
            vertexFormat : Cesium.VertexFormat.POSITION_AND_NORMAL
        }),
        attributes: {
            color: Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(0.0, 1.0, 0.0, 0.5))
        }
    }),
    appearance : new Cesium.PerInstanceColorAppearance({
        closed : true
    })
}));

[cesium-concierge]

Thanks for the pull request @hpinkos!

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[mramato]

Thanks @hpinkos! Please merge in master.

@bagnell I think you're most qualified to review this.

[bagnell]

This doesn't fix #1297. It doesn't crash, but the artifact is still there. Here is the updated code example:

var viewer = new Cesium.Viewer('cesiumContainer');
viewer.entities.add({
    rectangle : {
        coordinates : Cesium.Rectangle.fromDegrees(-180.0, 45.0, -160.0, 90.0),
        outline : true,
        outlineColor : Cesium.Color.WHITE,
        material : Cesium.Color.BLUE,
        height : 0.0
    }
});

The only difference between this code and that in the issue is height : 0. So for some reason a rectangle clamped to the ground is different from a normal rectangle geometry?

[hpinkos]

Gotcha. I remembered to set height: 0 when i was testing out some other things but I must have forgot to set it when I was testing that example

[hpinkos]

@bagnell merged in master. Anything else?

[bagnell]

No, this looks good. Merging.