Compositor: fix initial dirty rect with scaling < 1

src/Avalonia.Base/Rendering/Composition/Server/ServerCompositionTarget.cs

@@ -159,14 +159,15 @@ namespace Avalonia.Rendering.Composition.Server
             _redrawRequested = false;
             using (var targetContext = _renderTarget.CreateDrawingContext())
             {
-                var layerSize = Size * Scaling;
+                var size = Size;
+                var layerSize = size * Scaling;
                 if (layerSize != _layerSize || _layer == null || _layer.IsCorrupted)
                 {
                     _layer?.Dispose();
                     _layer = null;
-                    _layer = targetContext.CreateLayer(Size);
+                    _layer = targetContext.CreateLayer(size);
                     _layerSize = layerSize;
-                    _dirtyRect = new Rect(0, 0, layerSize.Width, layerSize.Height);
+                    _dirtyRect = new Rect(0, 0, size.Width, size.Height);
                 }
 
                 if (_dirtyRect.Width != 0 || _dirtyRect.Height != 0)
@@ -187,7 +188,7 @@ namespace Avalonia.Rendering.Composition.Server
                 else
                     targetContext.DrawBitmap(_layer, 1,
                         new Rect(_layerSize),
-                        new Rect(Size));
+                        new Rect(size));
 
                 if (DebugOverlays != RendererDebugOverlays.None)
                 {