Change roughness modifier to make puddles

[TokisanGames]

Description

Painting wetness introduces a roughness modifier that can increase or decrease roughness (-0.5, 0.5). Consider, instead of allowing rougher values, move the range to (-1, 0). (-0.5, 0) still increases wetness as is current. But below -0.5 have it flatten the normal to make puddles.

https://www.youtube.com/watch?v=DxOjiV-vNqM

Interpolate normals:
https://stackoverflow.com/questions/38070899/how-to-interpolate-normals-for-phong-shading-in-opengl
https://en.wikipedia.org/wiki/Slerp
https://hub.jmonkeyengine.org/t/normals-interpolation-in-shader/31598/14

[FishOfTheNorthStar]

I started a discussion about this concept recently, not the exact same implementation but pretty similar. Here's a link:
#375

There's an animated rain drop texture there I made that might come in handy.