Updated GenerateMask helper

Editor/AssetsHelpers/Enums/TextureChannel.cs

@@ -1,10 +1,31 @@
-namespace Misaki.ArtToolEditor
+using System;
+
+namespace Misaki.ArtToolEditor
 {
     internal enum TextureChannel
     {
+        None,
         R,
         G,
         B,
         A
     }
+
+    internal static class TextureChannelHelpers
+    {
+        private static readonly ArgumentException _channelNoneException = new("Channel is none");
+
+        public static int ToColorIndex(this TextureChannel channel)
+        {
+            return channel switch
+            {
+                TextureChannel.None => throw _channelNoneException,
+                TextureChannel.R => 0,
+                TextureChannel.G => 1,
+                TextureChannel.B => 2,
+                TextureChannel.A => 3,
+                _ => throw _channelNoneException
+            };
+        }
+    }
 }