Move the NRGBAColor interface to be with the new FromColor function, since that's where it's used.

oklab.go

@@ -106,6 +106,14 @@ func FromRGBA(r, g, b, a uint32) Color {
 	return FromNRGBA(r, g, b, a)
 }
 
+// NRGBAColor represent a color that can give us non pre-multiplied RGBA components.
+//
+// This isn't a standard interface, but we implement it and check for it regardless.
+type NRGBAColor interface {
+	color.Color
+	NRGBA() (r, g, b, a uint32)
+}
+
 // FromColor converts an arbitrary color type to an OKLab [Color], with special handling
 // for [color.NRGBA], [color.NRGBA64], and anything implementing the [NRGBAColor] interface
 // to preserve transparent colors.
@@ -172,14 +180,6 @@ func Distance(a, b Color) float64 {
 	return math.Sqrt(max(sqr(dL), sqr(dL+dA)) + max(sqr(da), sqr(da+dA)) + max(sqr(db), sqr(db+dA)))
 }
 
-// NRGBAColor represent a color that can give us non pre-multiplied RGBA components.
-//
-// This isn't a standard interface, but we implement it and check for it regardless.
-type NRGBAColor interface {
-	color.Color
-	NRGBA() (r, g, b, a uint32)
-}
-
 func okLabModel(c color.Color) color.Color {
 	return FromColor(c)
 }