Added CMYK to RGB unpacker

[radarhere]

Resolves #7309

Adds a CMYK to RGB unpacker, essentially copying the code from

Pillow/src/libImaging/Convert.c

Lines 567 to 579 in 07623d1

	static void
	cmyk2rgb(UINT8 *out, const UINT8 *in, int xsize) {
	int x, nk, tmp;
	for (x = 0; x < xsize; x++) {
	nk = 255 - in[3];
	out[0] = CLIP8(nk - MULDIV255(in[0], nk, tmp));
	out[1] = CLIP8(nk - MULDIV255(in[1], nk, tmp));
	out[2] = CLIP8(nk - MULDIV255(in[2], nk, tmp));
	out[3] = 255;
	out += 4;
	in += 4;
	}
	}

[hugovk]

essentially copying the code from ...

Is it worth de-duplicating, and having them both calling a common function?

[radarhere]

Ok, I've pushed a commit to include the Convert function in Unpack.