Add Points binding observability support for Polygon and Polyline

src/Avalonia.Controls/Shapes/Polygon.cs

@@ -12,6 +12,7 @@ namespace Avalonia.Controls.Shapes
         static Polygon()
         {
             AffectsGeometry<Polygon>(PointsProperty);
+            CollectionAffectsGeometry<Polygon>(PointsProperty);
         }
 
         public Polygon()

src/Avalonia.Controls/Shapes/Polyline.cs

@@ -13,6 +13,7 @@ namespace Avalonia.Controls.Shapes
         {
             StrokeThicknessProperty.OverrideDefaultValue<Polyline>(1);
             AffectsGeometry<Polyline>(PointsProperty);
+            CollectionAffectsGeometry<Polyline>(PointsProperty);
         }
 
         public Polyline()

src/Avalonia.Controls/Shapes/Shape.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Specialized;
 using Avalonia.Collections;
 using Avalonia.Media;
 using Avalonia.Media.Immutable;
@@ -219,6 +220,40 @@ namespace Avalonia.Controls.Shapes
                 });
             }
         }
+        
+        /// <summary>
+        /// Marks a property as a collection whose elements affecting the shape's geometry.
+        /// </summary>
+        /// <param name="properties">The properties.</param>
+        /// <remarks>
+        /// After a call to this method in a control's static constructor, any change to the
+        /// property will cause <see cref="InvalidateGeometry"/> to be called on the element.
+        /// </remarks>
+        protected static void CollectionAffectsGeometry<TShape>(params AvaloniaProperty[] properties)
+            where TShape : Shape
+        {
+            foreach (var property in properties)
+            {
+                property.Changed.Subscribe(e =>
+                {
+                    if (e.Sender is not TShape shape) return;
+                    if (e.OldValue is INotifyCollectionChanged oldCollection)
+                    {
+                        oldCollection.CollectionChanged -= shape.OnCollectionThatAffectsGeometryChanged;
+                    }
+                    
+                    if (e.NewValue is INotifyCollectionChanged newCollection)
+                    {
+                        newCollection.CollectionChanged += shape.OnCollectionThatAffectsGeometryChanged;
+                    }
+                });
+            }
+        }
+        
+        private void OnCollectionThatAffectsGeometryChanged(object? sender, NotifyCollectionChangedEventArgs e)
+        {
+            InvalidateGeometry();
+        }
 
         /// <summary>
         /// Creates the shape's defining geometry.