[css-backgrounds] <color> in <shadow> should default to currentcolor #2766

Currently the spec says:

Specifies the color of the shadow. If the color is absent, the used color is taken from the color property.

It seems to behave like currentcolor nowadays, but it would have different behavior in terms of interpolating. I think we should probably make it default to currentcolor instead. I see no reason we need a separate no-color value case.

(It's not clear to me whether it should go to level 3 or 4.)