Add test and fix for asynchronous streams.

src/ImageSharp/Image.FromStream.cs

@@ -588,7 +588,20 @@ public abstract partial class Image
         await stream.CopyToAsync(memoryStream, configuration.StreamProcessingBufferSize, cancellationToken).ConfigureAwait(false);
         memoryStream.Position = 0;
 
-        return action(memoryStream, cancellationToken);
+        T Action(Stream ms, CancellationToken ct)
+        {
+            // Reset the position of the seekable stream if we did not read to the end
+            // to allow additional reads.
+            T result = action(ms, ct);
+            if (stream.CanSeek && ms.Position != ms.Length)
+            {
+                stream.Position = ms.Position;
+            }
+
+            return result;
+        }
+
+        return Action(memoryStream, cancellationToken);
     }
 
     [DoesNotReturn]

tests/ImageSharp.Tests/Formats/GeneralFormatTests.cs

@@ -61,6 +61,21 @@ public class GeneralFormatTests
         Assert.NotEqual(i[5, 5], j[5, 5]);
     }
 
+    [Fact]
+    public async Task ReadOriginIsRespectedOnLoadAsync()
+    {
+        using FileStream stream = File.OpenRead(TestFile.GetInputFileFullPath(TestImages.Png.Issue2259));
+        using Image<Rgb24> i = await Image.LoadAsync<Rgb24>(stream);
+        long position1 = stream.Position;
+        Assert.NotEqual(0, position1);
+
+        using Image<Rgb24> j = await Image.LoadAsync<Rgb24>(stream);
+        long position2 = stream.Position;
+        Assert.True(position2 > position1);
+
+        Assert.NotEqual(i[5, 5], j[5, 5]);
+    }
+
     [Fact]
     public void ImageCanEncodeToString()
     {