specular_radius_scale should maybe be a color (not a vector3)?

[portsmouth]

Currently we say that subsurface_radius_scale has type vector3. But what does this mean operationally? It's confusing for people doing UI, as to whether this parameter should have a color picker, but we certainly want it to. The components are in [0,1] range so there's no issue with the values.

Perhaps it just means that the parameter should not be color managed, but do we not want that, as the meaning of the RGB channels (i.e. which "curve" of wavelength bands a given radius should apply to) should also change as the color space changes? So i'm not sure it doesn't make more sense to just leave it as a color.