fix(mix): Fix algorithm directly instead of returning early

Correctly handles the edge cases of bug #408. Based on the updated implementation in dart-sass: https://github.com/sass/dart-sass/blob/169178af6c6414d01bc66144b437701057ff3684/lib/src/functions/color.dart#L800-L838

src/mix.ts

@@ -5,9 +5,6 @@ import rgba from './rgba';
  * Mixes two colors together. Taken from sass's implementation.
  */
 function mix(color1: string, color2: string, weight: number): string {
-  if (weight <= 0) return rgba(...parseToRgba(color1));
-  if (weight >= 1) return rgba(...parseToRgba(color2));
-
   const normalize = (n: number, index: number) =>
     // 3rd index is alpha channel which is already normalized
     index === 3 ? n : n / 255;
@@ -18,10 +15,12 @@ function mix(color1: string, color2: string, weight: number): string {
   // The formula is copied from the original Sass implementation:
   // http://sass-lang.com/documentation/Sass/Script/Functions.html#mix-instance_method
   const alphaDelta = a2 - a1;
-  const x = weight * 2 - 1;
-  const y = x * alphaDelta === -1 ? x : x + alphaDelta;
-  const z = 1 + x * alphaDelta;
-  const weight2 = (y / z + 1) / 2.0;
+  const normalizedWeight = weight * 2 - 1;
+  const combinedWeight =
+    normalizedWeight * alphaDelta === -1
+      ? normalizedWeight
+      : normalizedWeight + alphaDelta / (1 + normalizedWeight * alphaDelta);
+  const weight2 = (combinedWeight + 1) / 2;
   const weight1 = 1 - weight2;
 
   const r = (r1 * weight1 + r2 * weight2) * 255;