Add enter portals example

[pschroen]

References:

• https://twitter.com/0xca0a/status/1669698823931809795
• https://blog.maximeheckel.com/posts/beautiful-and-mind-bending-effects-with-webgl-render-targets/
• https://twitter.com/jesper_vos/status/1588496087429808128
• https://medium.com/resn/bandit%C3%B8-immersive-experience-f258361dead3

[pschroen]

cc @cyrstem 😉

[Rishi-Sudhakar]

Wow, looks so dope. This would be really awesome.