Artifact appears on polygon when viewed from a low angle



angle of view is lower, the shadow will appear，i want the shadow disappear

this is code:
```js
var viewer = new Cesium.Viewer("cesiumContainer");
viewer.scene.globe.enableLighting = false;
viewer.shadows = false;
var redPolygon = viewer.entities.add({
name: "Red polygon on surface",
polygon: {
hierarchy: Cesium.Cartesian3.fromDegreesArrayHeights([
116.31282326252641,
40.02362677720817,
-2.502101525,
116.31980360345781,
40.02362677720817,
-2.502101525,
116.31980360345781,
40.012985336043336,
-2.502101525,
116.31282326252641,
40.012985336043336,
-2.502101525,
116.31282326252641,
40.02362677720817,
-2.502101525
]),
height: 1000,
material: Cesium.Color.RED,
},
});

viewer.zoomTo(viewer.entities);
```

Reproduce the artifact with the provided Cesium Viewer and polygon entity snippet, especially at a low camera angle with globe lighting and viewer shadows disabled. Start by tracing polygon rendering and shadow handling, then verify that the low-angle artifact is gone without changing the intended polygon appearance.

Written by the indexing model from the issue text.