DropShadowPanel Content wireup size changed

Wire up DropShadowPanel Content's size change to ensure that shadow is updated correctly.
Address #1111 and #1299

Author: hermitdave

Microsoft.Toolkit.Uwp.UI.Controls/DropShadowPanel/DropShadowPanel.cs

@@ -50,8 +50,6 @@ public DropShadowPanel()
 
                 _dropShadow = compositor.CreateDropShadow();
                 _shadowVisual.Shadow = _dropShadow;
-
-                SizeChanged += OnSizeChanged;
             }
         }
 
@@ -72,6 +70,13 @@ protected override void OnApplyTemplate()
                 ElementCompositionPreview.SetElementChildVisual(_border, _shadowVisual);
             }
 
+            var content = this.Content as FrameworkElement;
+
+            if (content != null)
+            {
+                content.SizeChanged += OnSizeChanged;
+            }
+
             ConfigureShadowVisualForCastingElement();
 
             base.OnApplyTemplate();