🐛 Fix pinning of memory mapped file buffers beyond the first byte.

This is the main reason for corruption of GIF images uploaded to Kraken Z. (Simply would not occur for images smaller than 2MB)

src/Exo/Core/Exo.Memory/MemoryMappedFileMemoryManager.cs

@@ -31,9 +31,10 @@ protected override void Dispose(bool disposing)
 
 	public override MemoryHandle Pin(int elementIndex = 0)
 	{
+		ArgumentOutOfRangeException.ThrowIfGreaterThan((uint)elementIndex, (uint)_length, nameof(elementIndex));
 		byte* pointer = null;
 		_viewHandle.AcquirePointer(ref pointer);
-		return new MemoryHandle(pointer, pinnable: this);
+		return new MemoryHandle(pointer + (uint)elementIndex, pinnable: this);
 	}
 
 	public override void Unpin() => _viewHandle.ReleasePointer();