ImageBrush shouldn't Dispose of the image it is using. (#883)

Fixes #881

src/ImageSharp.Drawing/Processing/ImageBrush{TPixel}.cs

@@ -113,7 +113,6 @@ namespace SixLabors.ImageSharp.Processing
             /// <inheritdoc />
             public override void Dispose()
             {
-                this.source.Dispose();
             }
 
             /// <inheritdoc />

tests/ImageSharp.Tests/Drawing/FillImageBrushTests.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Primitives;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
+using SixLabors.Primitives;
+using SixLabors.Shapes;
+
+using Xunit;
+
+// ReSharper disable InconsistentNaming
+
+namespace SixLabors.ImageSharp.Tests.Drawing
+{
+    [GroupOutput("Drawing")]
+    public class FillImageBrushTests
+    {
+        [Fact]
+        public void DoesNotDisposeImage()
+        {
+            using (var src = new Image<Rgba32>(5, 5))
+            {
+                var brush = new ImageBrush<Rgba32>(src);
+                using (var dest = new Image<Rgba32>(10, 10))
+                {
+                    dest.Mutate(c => c.Fill(brush, new Rectangle(0, 0, 10, 10)));
+                    dest.Mutate(c => c.Fill(brush, new Rectangle(0, 0, 10, 10)));
+                }
+            }
+        }
+    }
+}