Allow polylines to connect points with Rhumb lines

[ggetz]

var viewer = new Cesium.Viewer("cesiumContainer" );

viewer.entities.add({
    polyline: {
        positions: Cesium.Cartesian3.fromDegreesArray([-120, 20, 120, 20])
    }
});

Turn on 2D mode and the line will appear warped, even though it's connecting two points of a constant latitude, because the polyline is connecting the two points with a great arc.

The followSurface property only allows the two points to be connected with a great arc or linearly (which clips through the globe): https://cesiumjs.org/Cesium/Build/Documentation/PolylineGraphics.html?classFilter=polyline#followSurface

[hpinkos]

Duplicate #4000

[ggetz]

Thanks @hpinkos!