Consider updating Oklab matrices for better round-tripping

[danburzo]

The issue came up in #236 that sRGB / Oklch roundtripping is not perfect for achromatic colors. We are using the Oklab matrices from Björn Ottosson’s original article, but css-color-4 has updated them based on this discussion. The conversion code at the time of writing:

// OKLab and OKLCH
// https://bottosson.github.io/posts/oklab/

// XYZ <-> LMS matrices recalculated for consistent reference white
// see https://github.com/w3c/csswg-drafts/issues/6642#issuecomment-943521484
// recalculated for 64bit precision
// see https://github.com/color-js/color.js/pull/357

function XYZ_to_OKLab(XYZ) {
	// Given XYZ relative to D65, convert to OKLab
	var XYZtoLMS = [
		[ 0.8190224379967030, 0.3619062600528904, -0.1288737815209879 ],
		[ 0.0329836539323885, 0.9292868615863434,  0.0361446663506424 ],
		[ 0.0481771893596242, 0.2642395317527308,  0.6335478284694309 ]
	];
	var LMStoOKLab = [
		[ 0.2104542683093140,  0.7936177747023054, -0.0040720430116193 ],
		[ 1.9779985324311684, -2.4285922420485799,  0.4505937096174110 ],
		[ 0.0259040424655478,  0.7827717124575296, -0.8086757549230774 ]
	];

	var LMS = multiplyMatrices(XYZtoLMS, XYZ);
	// JavaScript Math.cbrt returns a sign-matched cube root
	// beware if porting to other languages
	// especially if tempted to use a general power function
	return multiplyMatrices(LMStoOKLab, LMS.map(c => Math.cbrt(c)));
	// L in range [0,1]. For use in CSS, multiply by 100 and add a percent
}

function OKLab_to_XYZ(OKLab) {
	// Given OKLab, convert to XYZ relative to D65
	var LMStoXYZ =  [
		[  1.2268798758459243, -0.5578149944602171,  0.2813910456659647 ],
		[ -0.0405757452148008,  1.1122868032803170, -0.0717110580655164 ],
		[ -0.0763729366746601, -0.4214933324022432,  1.5869240198367816 ]
	];
	var OKLabtoLMS = [
		[ 1.0000000000000000,  0.3963377773761749,  0.2158037573099136 ],
		[ 1.0000000000000000, -0.1055613458156586, -0.0638541728258133 ],
		[ 1.0000000000000000, -0.0894841775298119, -1.2914855480194092 ]
    ];

	var LMSnl = multiplyMatrices(OKLabtoLMS, OKLab);
	return multiplyMatrices(LMStoXYZ, LMSnl.map(c => c ** 3));
}

The updated matrices seem to result in improved round-tripping.