Rendering: add a Screen Space Subsurface Scattering pass
In order to render skin, or marble more realistically, we need to compute light diffusion under the surface (the scattering of light under the surface == Subsurface Scattering)
To achieve this, a simple and fast screen space algorithm exists, Screen Space Subsurface Scattering (SSSS) : see https://therealmjp.github.io/posts/sss-intro/ or this good paper www.iryoku.com/sssss/downloads/Screen-Space-Perceptual-Rendering-of-Human-Skin.pdf
Technically, it works by a blurring the diffuse part with a gaussian kernel (and a diffusion profile).
Here is a WIP branch that adds this pass as a RenderPass, but needs more kernel tweaking