Ensure Bounds() is accurate per operation

src/ImageSharp/DefaultInternalImageProcessorContext.cs

@@ -47,7 +47,7 @@ namespace SixLabors.ImageSharp
         }
 
         /// <inheritdoc/>
-        public Rectangle Bounds() => this.source.Bounds();
+        public Rectangle Bounds() => this.destination?.Bounds() ?? this.source.Bounds();
 
         /// <inheritdoc/>
         public IImageProcessingContext<TPixel> ApplyProcessor(IImageProcessor<TPixel> processor, Rectangle rectangle)

tests/ImageSharp.Tests/Image/ImageProcessingContextTests.cs

@@ -0,0 +1,58 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.Primitives;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests
+{
+    public class ImageProcessingContextTests
+    {
+        [Fact]
+        public void MutatedBoundsAreAccuratePerOperation()
+        {
+            var x500 = new Size(500, 500);
+            var x400 = new Size(400, 400);
+            var x300 = new Size(300, 300);
+            var x200 = new Size(200, 200);
+            var x100 = new Size(100, 100);
+            using (var image = new Image<Rgba32>(500, 500))
+            {
+                image.Mutate(x =>
+                    x.AssertBounds(x500)
+                        .Resize(x400).AssertBounds(x400)
+                        .Resize(x300).AssertBounds(x300)
+                        .Resize(x200).AssertBounds(x200)
+                        .Resize(x100).AssertBounds(x100));
+            }
+        }
+
+        [Fact]
+        public void ClonedBoundsAreAccuratePerOperation()
+        {
+            var x500 = new Size(500, 500);
+            var x400 = new Size(400, 400);
+            var x300 = new Size(300, 300);
+            var x200 = new Size(200, 200);
+            var x100 = new Size(100, 100);
+            using (var image = new Image<Rgba32>(500, 500))
+            {
+                image.Clone(x =>
+                    x.AssertBounds(x500)
+                        .Resize(x400).AssertBounds(x400)
+                        .Resize(x300).AssertBounds(x300)
+                        .Resize(x200).AssertBounds(x200)
+                        .Resize(x100).AssertBounds(x100));
+            }
+        }
+    }
+
+    public static class BoundsAssertationExtensions
+    {
+        public static IImageProcessingContext<Rgba32> AssertBounds(this IImageProcessingContext<Rgba32> context, Size size)
+        {
+            Assert.Equal(size, context.Bounds().Size);
+            return context;
+        }
+    }
+}