Improve screen space subsurface scattering for vegetation

[PanosK92]

We currently have a basic implementation of screen space subsurface scattering (SSS) specifically designed for vegetation. While it serves its fundamental purpose, there's significant room for improvement. This issue aims to explore ways to enhance our current SSS approach, making it more robust and visually appealing.

Without sss:

With the existing sss:

Source: https://github.com/PanosK92/SpartanEngine/blob/master/data/shaders/light.hlsl