Add license headers.

src/Skia/Avalonia.Skia/CustomRenderTarget.cs

@@ -1,3 +1,6 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
 using Avalonia.Platform;
 using Avalonia.Rendering;
 

src/Skia/Avalonia.Skia/ICustomSkiaGpu.cs

@@ -1,3 +1,6 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
 using System.Collections.Generic;
 using SkiaSharp;
 

src/Skia/Avalonia.Skia/ICustomSkiaRenderSession.cs

@@ -1,3 +1,6 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
 using System;
 using SkiaSharp;
 

src/Skia/Avalonia.Skia/ICustomSkiaRenderTarget.cs

@@ -1,3 +1,6 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
 using System;
 
 namespace Avalonia.Skia