3 shaders(more to come) on the theme of layering.

update 1.1:

-mask can now use uv1 channel
-new compact foldable gui for masked layer shader.

features surface shaders that:
- blend two layers with mask
- blend three layers with their alpha

*("layer" is combination of color, normal, and metallic-smoothness maps)

features unlit shader that:
- blends up to 6 layers with their alpha