Path Geometry update fix for 4748 bug rerender on change for paths segments (#18025)

* Add failing Path Geometry update test

#4748 [BUG] Rerender on change for Paths, Segments and e.g.
Failing Test

* Fixes failing Path Geometry update test

Fixes #4748 [BUG] Rerender on change for Paths, Segments and e.g.

src/Avalonia.Base/Media/PathGeometry.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Media
         /// </summary>
         public PathGeometry()
         {
-            _figures = new PathFigures();
+            Figures = new PathFigures();
         }
 
         /// <summary>

tests/Avalonia.Base.UnitTests/Media/PathGeometryTests.cs

@@ -0,0 +1,32 @@
+using Avalonia.Media;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Media;
+
+public class PathGeometryTests
+{
+    [Fact]
+    public void PathGeometry_Triggers_Invalidation_On_Figures_Add()
+    {
+        var segment = new PolyLineSegment()
+        {
+            Points = [new Point(1, 1), new Point(2, 2)]
+        };
+
+        var figure = new PathFigure()
+        {
+            Segments = [segment],
+            IsClosed = false,
+            IsFilled = false,
+        };
+        
+        var target = new PathGeometry();
+
+        var changed = false;
+
+        target.Changed += (_, _) => { changed = true; };
+        
+        target.Figures?.Add(figure);
+        Assert.True(changed);
+    }
+}