ND_surface should have color opacity

Currently opacity is defined as a `float` in `ND_surface`. There are many use cases (stained glass) where it would be good to have opacity defined as a `color3`.

There's even a documented work-around in `standard_surface.mtlx` to convert the incoming color to a scalar value. Full color opacity is needed to reproduce the standard surface faithfully.
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It's easy to get a scalar value out of a color, it's not possible to get a color out of a scalar.