Support for Nishita Sky Texture

[mercielgo]

Please add Nishita Sky Texture into Luxcore.

[bestman8]

https://forums.luxcorerender.org/viewtopic.php?f=4&t=2552 maybe take a look at that first. but i do think that a better sky model can be possible there are a lot of good ones. i personally like the ones i mentioned below but i based that on this https://arxiv.org/pdf/1612.04336.pdf and just the looks so it is fully possible that they are bad or wont even be possible in luxcore / any pathtracer http://www.klayge.org/material/4_0/Atmospheric/Real-Time%20Spectral%20Scattering%20in%20Large-Scale%20Natural%20Participating%20Media.pdf https://hal.inria.fr/file/index/docid/288758/filename/article.pdf

[EntityinArray]

luxcore's hosek-wilkie sky is dark and washed out. I wanted Nishita because in Cycles it looks amazing by default