Add clouds

[PanosK92]

Atmospheric scattering is there, but there is an obvious missing piece, clouds.

There are multiple ways to go about rendering clouds but I think one of the most common ways is using algorithms like Simplex.

They don't have to be volumetric, they can simply be cloud like shapes generated from the atmospheric scattering shader: https://github.com/PanosK92/SpartanEngine/blob/master/data/shaders/atmospheric_scattering.hlsl