Multistep functions

multistep functions

```glsl
float mix4smoothstep(float a, float b, float c, float d, float x ) {
    return mix(mix(a,b,smoothstep(0.,.33,x)),
               mix(b,
                   mix(c,d,smoothstep(.66,.99,x)),
                   smoothstep(.33,.66,x)),
               step(.33,x));
}

float mix4linear(float a, float b, float c, float d, float x ) {
    return mix(mix(a,b,clamp(x,0.,.33)*3.),
               mix(b,
                   mix(c,d,(clamp(x,.66,1.)-.66)*3.),
                   (clamp(x,.33,.66)-.33)*3.),
               step(0.33,x));
}
```