Ensure ComManagedStream seekable wrapper stream has position of 0 (#12953)

Until .NET 5, `ComManagedStream` (then `GPStream`) would wrap a
non-seekable stream in a `MemoryStream` using the `MemoryStream(Byte[])`
constructor. This results in a stream with a position of 0.

dotnet/runtime commit 136527537e6 (Improve perfromance for loading from
Stream on Windows (dotnet/corefx#31142), 2018-07-20) updated this logic
to instead use `CopyTo` to populate the wrapping `MemoryStream`. This
results in a stream with a non-zero position.

It seems that this non-zero position causes some issues in downstream
code when using `Image.FromStream` to load `.emf` and `.wmf` files,
resulting in `LoadGdipImageFromStream` returning a status of 2 and thus
an exception.

Seek the wrapping `MemoryStream` back to the beginning after copying the
source stream into it to prevent this exception.

Fixes #12951

Author: zivarah

src/System.Private.Windows.Core/src/Windows/Win32/System/Com/ComManagedStream.cs

@@ -19,6 +19,7 @@ internal ComManagedStream(Stream stream, bool makeSeekable = false)
             // Copy to a memory stream so we can seek
             MemoryStream memoryStream = new();
             stream.CopyTo(memoryStream);
+            memoryStream.Seek(0, SeekOrigin.Begin);
             _dataStream = memoryStream;
         }
         else

src/System.Private.Windows.Core/tests/System.Private.Windows.Core.Tests/Windows/Win32/System/Com/ComManagedStreamTests.cs

@@ -0,0 +1,36 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Windows.Win32.System.Com.Tests;
+
+public class ComManagedStreamTests
+{
+    [Fact]
+    public void Ctor_NonSeekableStream_WrapsWithSeekableStreamAtPositionZero()
+    {
+        using TestStream nonSeekableStream = new(canSeek: false, numBytes: 4);
+        ComManagedStream comManagedStream = new(nonSeekableStream, makeSeekable: true);
+        comManagedStream.GetDataStream().CanSeek.Should().Be(true);
+        comManagedStream.GetDataStream().Position.Should().Be(0);
+    }
+
+    [Fact]
+    public void Ctor_SeekableStream_UsesOriginalStream()
+    {
+        using TestStream seekableStream = new(canSeek: true, numBytes: 4);
+        ComManagedStream comManagedStream = new(seekableStream, makeSeekable: true);
+        comManagedStream.GetDataStream().Should().BeSameAs(seekableStream);
+    }
+
+    private class TestStream : MemoryStream
+    {
+        private readonly bool _canSeek;
+
+        public override bool CanSeek => _canSeek;
+
+        public TestStream(bool canSeek, int numBytes) : base(new byte[numBytes])
+        {
+            _canSeek = canSeek;
+        }
+    }
+}