Merge pull request #2748 from MarchingCube/fix-renderlayers-clear

Fix RenderLayers.Clear not clearing inner collection

src/Avalonia.Visuals/Rendering/RenderLayers.cs

@@ -8,8 +8,8 @@ namespace Avalonia.Rendering
 {
     public class RenderLayers : IEnumerable<RenderLayer>
     {
-        private List<RenderLayer> _inner = new List<RenderLayer>();
-        private Dictionary<IVisual, RenderLayer> _index = new Dictionary<IVisual, RenderLayer>();
+        private readonly List<RenderLayer> _inner = new List<RenderLayer>();
+        private readonly Dictionary<IVisual, RenderLayer> _index = new Dictionary<IVisual, RenderLayer>();
         
         public int Count => _inner.Count;
         public RenderLayer this[IVisual layerRoot] => _index[layerRoot];
@@ -56,6 +56,7 @@ namespace Avalonia.Rendering
             }
 
             _index.Clear();
+            _inner.Clear();
         }
 
         public bool TryGetValue(IVisual layerRoot, out RenderLayer value)