Fix webgl_gpgpu_birds references error (#20675)
* fix_webgl_gpgpu_birds_index_error
* screenshot
* Restore Left Right names
* Update webgl_gpgpu_birds.html
Co-authored-by: NMichael Herzog <michael.herzog@human-interactive.org>
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![examples/screenshots/webgl_gpgpu_birds.jpg](https://gitcode.net/qq_44235792/three.js/-/raw/1dd66f3962e7a63a2dbf0cd8f6790c03aa34724b/examples/screenshots/webgl_gpgpu_birds.jpg)
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![examples/screenshots/webgl_gpgpu_birds.jpg](https://gitcode.net/qq_44235792/three.js/-/raw/1dd66f3962e7a63a2dbf0cd8f6790c03aa34724b/examples/screenshots/webgl_gpgpu_birds.jpg)
![examples/screenshots/webgl_gpgpu_birds.jpg](https://gitcode.net/qq_44235792/three.js/-/raw/1dd66f3962e7a63a2dbf0cd8f6790c03aa34724b/examples/screenshots/webgl_gpgpu_birds.jpg)