Provide lighter versions of the procedural skin shaders

[joepal1976]

The "layered" skin shader is currently extra-everything. In many use cases this is unneccessarily heavy, making rendering and viewport handling slow. Further, it will make machines with low-end graphics cards crash.

There should be limited versions of the shader, where seldomly used things such as special groups for ears, aureolae, genitals and toenails are disabled.

While at this, the "enhanced" shader should be revisited too, to get the same treatment.