Error loading pointcloud tiles on WebGL: Uncaught RuntimeError: memory access out of bounds

### What happened?

With 1.20 we got experimental WebGL support. Works great for Worldterrain and OSM buildings, but tilesets with pointclouds are not loading, because of a runtime error:

Uncaught RuntimeError: memory access out of bounds

We encountered this with our own dataset, but it can be easily reproduced with the sample project.

### Environment

Cesium For Unity Version: 1.20
Unity Editor Version: 6.0.58f2 (Used to open cesium samples)
Operation System: Windows 11 with RTX2070 Super
Can the issue be reproduced with cesium samples: Yes
When does the issue occur: Building the point cloud sample for the web platform and running in Chrome
Renderpipeline: URP-HighFidelity
Stacktrace: [Stacktrace.txt](https://github.com/user-attachments/files/23939288/Stacktrace.txt)

### Reproduction steps

1. Open the cesium samples project
2. Configure the build:
2.1 Set target to WebGL
2.2 Set 06_CesiumPointClouds as the only scene for the build
2.3 In the WebGL playersettings, set "Enable Native C/C++ Multithreading" to true
2.4 Optional: Enable debug settings
- Enable development build
- Set Enable Exceptions to "Full With Stacktrace"
- Enable debug symbols
3. Built the WebGL application
4. Use a HTTP server which supports both COOP and COEP, which is required for multithreading. Built and run does not work.
5. Load the website and wait for the scene to load
6. The application encountered a runtime error: Uncaught RuntimeError: memory access out of bounds

### Supporting evidence

_No response_