Rename medium entity SSS to strong and add new medium preset

[MikiP98]

OPTION B: Describe the feature you would like to be changed in the shader

Current medium SSS for entities is pretty strong, while the current low SSS for entities is barely visible is you look closely on a white entity.
I increased the value of low from 0.3 to 0.4 and it now is visible on a non-white entities, but it's still pretty weak, and the medium SSS of 0.75 is too strong.
A new medium with a value of 0.5 or 0.6 would be nice for many modded entities.

I would imagine new values to be:

• strong: 0.75-1.0 (When introducing new medium, the current strong can be boosted a bit)
• medium: 0.5-0.65 (New medium, good for example for naturalist:snail)
• weak: 0.35-0.4 (Slightly boosted weak SSS, so that it's at least visible)