Add simple photon caustics

[Juraj]

duplicate (more general): https://forum.corona-renderer.com/index.php/topic,10923.0.html

That's accidentally a link to my thread :- ) ? I guess a typo

[Ondra]

nope - you also want caustics, right?

[Juraj]

I do, but that thread is about the bug that happens with refractive materials, although the issue manifests in reflection. I took the name from the Corona Manual and added more info to the thread to avoid confusion.

Shouldn't have been so sparse in description, sorry.