Flickering rendering artifacts near Cartographic Polygon clipping on macOS / iOS

### What happened?

## Description
As shown in the screenshot, visual artifacts appear near the clipped area when using **Cesium Cartographic Polygons**.

The artifacts are **not static**; their positions **shift every frame**, resulting in flickering and unstable rendering.

This issue occurs on **macOS (Unity Editor)** and **iOS (built app)**, but **does not occur on Windows**.

## Expected behavior
The clipped area should render cleanly and consistently, without flickering or visual artifacts, as it does on Windows.

## Actual behavior
Flickering / unstable rendering artifacts appear around the clipping boundary, especially near the ground and building edges.

### Environment

- Cesium for Unity version: 1.18.1 – 1.20.0
- Unity Editor version: 6000.2.10f1
- Hardware: Mac mini (2023)
- OS: macOS Tahoe 26.0.1
- Reproduced on:
- macOS (Unity Editor)
- iPhone 12 Pro (iOS build)
- Not reproduced on:
- Windows

### Reproduction steps

The issue can be reproduced in a minimal scene by following the official clipping documentation:
https://cesium.com/learn/unity/unity-clipping/

1. Create a new default Unity scene
2. Add **Google Photorealistic 3D Tiles** via the Cesium menu
3. Install **Splines** from the Package Manager
4. Attach **CesiumCartographicPolygon** and **CesiumPolygonRasterOverlay** to the Cesium3DTileset
5. Assign the polygon reference and enable clipping

### Supporting evidence

## Additional information
A similar rendering issue has been reported in **Cesium for Unreal**, which may indicate a platform-specific issue related to Apple GPUs / Metal:
https://github.com/CesiumGS/cesium-unreal/issues/1196