Added ChannelMixer

Editor/AssetsHelpers/Helpers/TextureHelpers.cs

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+using System.IO;
+using UnityEngine;
+using UnityEngine.Experimental.Rendering;
+
+namespace Misaki.ArtToolEditor
+{
+    internal static class TextureHelpers
+    {
+        internal static void ExportRenderTextureToPNG(RenderTexture targetRT, string textureFullPath)
+        {
+            var exportTexture = new Texture2D(targetRT.width, targetRT.height, targetRT.graphicsFormat, TextureCreationFlags.None);
+            exportTexture.ReadPixels(new Rect(0, 0, targetRT.width, targetRT.height), 0, 0);
+            exportTexture.Apply();
+            var bytes = exportTexture.EncodeToPNG();
+            File.WriteAllBytes(textureFullPath, bytes);
+
+            UnityEngine.Object.DestroyImmediate(exportTexture);
+        }
+    }
+}

Editor/AssetsHelpers/Helpers/TextureHelpers.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: 44fc0bf1b58d77b47ac2af048b958418