Add support for custom shadow alpha masks

Author: arcadiogarcia

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

@@ -2,9 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using Microsoft.Toolkit.Uwp.UI.Controls.Core.DropShadowPanel;
 using System;
 using System.Numerics;
-using System.Threading.Tasks;
 using Windows.UI;
 using Windows.UI.Composition;
 using Windows.UI.Xaml;
@@ -168,7 +168,11 @@ private void UpdateShadowMask()
             {
                 CompositionBrush mask = null;
 
-                if (Content is Image)
+                if (Content is IAlphaMaskProvider maskedControl)
+                {
+                    mask = maskedControl.GetAlphaMask();
+                }
+                else if (Content is Image)
                 {
                     mask = ((Image)Content).GetAlphaMask();
                 }

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

@@ -0,0 +1,16 @@
+using Windows.UI.Composition;
+
+namespace Microsoft.Toolkit.Uwp.UI.Controls.Core.DropShadowPanel
+{
+    /// <summary>
+    /// Any user control can implement this interface to provide a custom alpha mask to it's parent <see cref="DropShadowPanel"/>
+    /// </summary>
+    public interface IAlphaMaskProvider
+    {
+        /// <summary>
+        /// This method should return the appropiate alpha mask to be used in the shadow of this control
+        /// </summary>
+        /// <returns>The alpha mask as a composition brush</returns>
+        CompositionBrush GetAlphaMask();
+    }
+}