Update Buffer2D to allow disposal testing.

src/ImageSharp/Memory/Buffer2D{T}.cs

@@ -55,6 +55,8 @@ public sealed class Buffer2D<T> : IDisposable
     /// </remarks>
     internal MemoryGroup<T> FastMemoryGroup { get; private set; }
 
+    internal bool IsDisposed { get; private set; }
+
     /// <summary>
     /// Gets a reference to the element at the specified position.
     /// </summary>
@@ -79,7 +81,11 @@ public sealed class Buffer2D<T> : IDisposable
     /// <summary>
     /// Disposes the <see cref="Buffer2D{T}"/> instance
     /// </summary>
-    public void Dispose() => this.FastMemoryGroup.Dispose();
+    public void Dispose()
+    {
+        this.FastMemoryGroup.Dispose();
+        this.IsDisposed = true;
+    }
 
     /// <summary>
     /// Gets a <see cref="Span{T}"/> to the row 'y' beginning from the pixel at the first pixel on that row.

tests/ImageSharp.Tests/Image/ImageFrameCollectionTests.Generic.cs

@@ -182,12 +182,12 @@ public abstract partial class ImageFrameCollectionTests
                 new[] { imageFrame1, imageFrame2 });
 
             IPixelSource<Rgba32>[] framesSnapShot = collection.OfType<IPixelSource<Rgba32>>().ToArray();
+
+            Assert.All(framesSnapShot, f => Assert.False(f.PixelBuffer.IsDisposed));
+
             collection.Dispose();
 
-            Assert.All(
-                framesSnapShot,
-                f => // The pixel source of the frame is null after its been disposed.
-                    Assert.Null(f.PixelBuffer));
+            Assert.All(framesSnapShot, f => Assert.True(f.PixelBuffer.IsDisposed));
         }
 
         [Theory]