Potential Photosensitive epilepsy at demo pages

Describe the bug

This demo page contains extremely strong blinks and flashes, which may cause a potential Photosensitive epilepsy. Better to add a warning at the entry of this demo.

To Reproduce

Steps to reproduce the behavior:

1. Go to ‘https://threejs.org/examples/webgl_postprocessing_glitch‘

Code

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Live example

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Expected behavior

Add a warning at the entry of this demo page.

Screenshots

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Platform:

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