Use WithW

Author: JimBobSquarePants

src/ImageSharp/PixelFormats/PixelBlenders/PorterDuffFunctions.cs

@@ -278,9 +278,7 @@ public static Vector4 Over(Vector4 destination, Vector4 source, Vector4 blend)
 
         // unpremultiply
         color /= Vector4.Max(alpha, new(Constants.Epsilon));
-        color.W = alpha.W;
-
-        return color;
+        return WithW(color, alpha);
     }
 
     /// <summary>
@@ -339,9 +337,7 @@ public static Vector4 Atop(Vector4 destination, Vector4 source, Vector4 blend)
 
         // unpremultiply
         color /= Vector4.Max(alpha, new(Constants.Epsilon));
-        color.W = alpha.W;
-
-        return color;
+        return WithW(color, alpha);
     }
 
     /// <summary>
@@ -385,9 +381,7 @@ public static Vector4 In(Vector4 destination, Vector4 source)
 
         Vector4 color = source * alpha;                    // premultiply
         color /= Vector4.Max(alpha, new(Constants.Epsilon));   // unpremultiply
-        color.W = alpha.W;
-
-        return color;
+        return WithW(color, alpha);
     }
 
     /// <summary>
@@ -425,9 +419,7 @@ public static Vector4 Out(Vector4 destination, Vector4 source)
 
         Vector4 color = source * alpha;                    // premultiply
         color /= Vector4.Max(alpha, new(Constants.Epsilon));   // unpremultiply
-        color.W = alpha.W;
-
-        return color;
+        return WithW(color, alpha);
     }
 
     /// <summary>
@@ -470,9 +462,7 @@ public static Vector4 Xor(Vector4 destination, Vector4 source)
 
         // unpremultiply
         color /= Vector4.Max(alpha, new(Constants.Epsilon));
-        color.W = alpha.W;
-
-        return color;
+        return WithW(color, alpha);
     }
 
     /// <summary>