RGBA() does not return the right values

[billy4479]

Right now the color.RBGA() function is implemented as

// RGBA implements color.Color
func (c Color) RGBA() (r uint32, g uint32, b uint32, a uint32) {
	return c.RGB[0], c.RGB[1], c.RGB[2], 1
}

therefore the values returned are in the interval [0,0xff].

This behavior is different from the one of image.color.RGBA.RGBA() which (as of go 1.23) implements it as

func (c RGBA) RGBA() (r, g, b, a uint32) {
	r = uint32(c.R)
	r |= r << 8
	g = uint32(c.G)
	g |= g << 8
	b = uint32(c.B)
	b |= b << 8
	a = uint32(c.A)
	a |= a << 8
	return
}

Indeed the image.color.Color interface is defined as

type Color interface {
	// RGBA returns the alpha-premultiplied red, green, blue and alpha values
	// for the color. Each value ranges within [0, 0xffff], but is represented
	// by a uint32 so that multiplying by a blend factor up to 0xffff will not
	// overflow.
	//
	// An alpha-premultiplied color component c has been scaled by alpha (a),
	// so has valid values 0 <= c <= a.
	RGBA() (r, g, b, a uint32)
}

therefore it expects that RGBA() returns a value in [0, 0xffff].

Even though we should implement the interface correctly, I still think it would be useful to be able to access the uint8 values, maybe with a RGBA8() function?