Revert ab29ec8: container==content auto-fit made it worse — back to alpha-crop with the container at element size

The container tracks the element's own Width/Height again; the crop stays in the bitmap.

MainWindow.xaml.cs

@@ -572,7 +572,6 @@ public partial class MainWindow : Window
         {
             _isResizing = true;
             _resizeAspect = sel.ClipShape == ClipShape.Round ? 1 : sel.Width / Math.Max(1, sel.Height);
-            _viewModel.IsPreviewDragging = true;
             grid.CaptureMouse();
             e.Handled = true;
             return;
@@ -584,7 +583,6 @@ public partial class MainWindow : Window
             _viewModel.SelectedElement = hit;
             _isDraggingOverlay = true;
             _grabOffset = new Point(p.X - hit.X, p.Y - hit.Y);
-            _viewModel.IsPreviewDragging = true;
             grid.CaptureMouse();
             e.Handled = true;
             return;
@@ -645,7 +643,6 @@ public partial class MainWindow : Window
             grid.ReleaseMouseCapture();
         _isDraggingOverlay = false;
         _isResizing = false;
-        _viewModel.IsPreviewDragging = false;
     }
 
     private bool HitHandle(Point mouseScreen, SceneElement element)