[css-color-4] w3c#10695 Fix rgbToHwb to use subset of rgbToHsl that only computes the hue

Tests related to out-of-gamut relative colors [css-color-5] already implicitly match the corrected behavior.

Author: squelart

css-color-4/rgbToHwb.js

@@ -1,12 +1,43 @@
+/**
+ * @param {number} red - Red component 0..1
+ * @param {number} green - Green component 0..1
+ * @param {number} blue - Blue component 0..1
+ * @return {number} Hue as degrees 0..360
+ */
+function rgbToHue(red, green, blue) {
+    // Similar to rgbToHsl, except that saturation and lightness are not calculated, and
+    // potential negative saturation is ignored.
+    let max = Math.max(red, green, blue);
+    let min = Math.min(red, green, blue);
+    let hue = NaN;
+    let d = max - min;
+
+    if (d !== 0) {
+        switch (max) {
+            case red:   hue = (green - blue) / d + (green < blue ? 6 : 0); break;
+            case green: hue = (blue - red) / d + 2; break;
+            case blue:  hue = (red - green) / d + 4;
+        }
+
+        hue *= 60;
+    }
+
+    if (hue >= 360) {
+        hue -= 360;
+    }
+
+    return hue;
+}
+
 /**
  * @param {number} red - Red component 0..1
  * @param {number} green - Green component 0..1
  * @param {number} blue - Blue component 0..1
  * @return {number[]} Array of HWB values: Hue as degrees 0..360, Whiteness and Blackness in reference range [0,100]
  */
 function rgbToHwb(red, green, blue) {
-    var hsl = rgbToHsl(red, green, blue);
+    var hue = rgbToHue(red, green, blue);
     var white = Math.min(red, green, blue);
     var black = 1 - Math.max(red, green, blue);
-    return([hsl[0], white*100, black*100]);
-}
+    return([hue, white*100, black*100]);
+}