[css-color-hdr] Handle negative component values in HLG function

Author: svgeesus

css-color-hdr/Overview.bs

@@ -555,8 +555,11 @@ Predefined color spaces for HDR: {#predefined-HDR}
 		const a = 0.17883277;
 		const b = 0.28466892;   // 1 - (4 * a)
 		const c = 0.55991073;   // 0.5 - a * Math.log(4 *a)
-		if (E <= 1/12) {
-			Edash = Math.sqrt( 3 * E);
+		// handle negative values
+		var sign = E < 0? -1 : 1;
+		var abs = Math.abs(E);
+		if (abs <= 1/12) {
+			Edash = sign * Math.sqrt( 3 * abs);
 		}
 		else {
 			Edash = a * Math.log(12 * E - b) + c;