feature: supports to view image diff when lfs object points to a image

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/LFSImageDiff.cs

@@ -0,0 +1,49 @@
+using System.Threading.Tasks;
+
+using Avalonia.Media.Imaging;
+using Avalonia.Threading;
+
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace SourceGit.ViewModels
+{
+    public class LFSImageDiff : ObservableObject
+    {
+        public Models.LFSDiff LFS
+        {
+            get;
+        }
+
+        public Models.ImageDiff Image
+        {
+            get => _image;
+            private set => SetProperty(ref _image, value);
+        }
+
+        public LFSImageDiff(string repo, Models.LFSDiff lfs)
+        {
+            LFS = lfs;
+
+            Task.Run(() =>
+            {
+                var img = new Models.ImageDiff();
+                (img.Old, img.OldFileSize) = BitmapFromLFSObject(repo, lfs.Old);
+                (img.New, img.NewFileSize) = BitmapFromLFSObject(repo, lfs.New);
+
+                Dispatcher.UIThread.Invoke(() => Image = img);
+            });
+        }
+
+        private (Bitmap, long) BitmapFromLFSObject(string repo, Models.LFSObject lfs)
+        {
+            if (string.IsNullOrEmpty(lfs.Oid) || lfs.Size == 0)
+                return (null, 0);
+
+            var stream = Commands.QueryFileContent.FromLFS(repo, lfs.Oid, lfs.Size);
+            var size = stream.Length;
+            return size > 0 ? (new Bitmap(stream), size) : (null, size);
+        }
+
+        private Models.ImageDiff _image;
+    }
+}