TAK: Fix return types for the color converter functions

Author: mficzel

Classes/Domain/ValueObject/HslaColor.php

@@ -146,7 +146,7 @@ private function hue2rgb(float $p, float $q, float $t): float
     /**
      * @return HslaColor
      */
-    public function asHsla(): self
+    public function asHsla(): HslaColor
     {
         return $this;
     }

Classes/Domain/ValueObject/RgbaColor.php

@@ -92,7 +92,7 @@ public function getAlpha(): float
     /**
      * @return RgbaColor
      */
-    public function asRgba(): self
+    public function asRgba(): RgbaColor
     {
         return $this;
     }