Refactor render layers a bit.

We be sure of the required state at the start of rendering,

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -92,7 +92,7 @@ namespace Avalonia.Rendering
             {
                 if (scene.Id != _lastSceneId)
                 {
-                    _layers.RemoveUnused(scene);
+                    _layers.Update(scene);
                     RenderToLayers(scene);
 
                     if (DebugFramesPath != null)
@@ -141,7 +141,7 @@ namespace Avalonia.Rendering
             {
                 foreach (var layer in scene.Layers)
                 {
-                    var renderTarget = GetRenderTargetForLayer(scene, layer.LayerRoot);
+                    var renderTarget = _layers[layer.LayerRoot].Bitmap;
                     var node = (VisualNode)scene.FindNode(layer.LayerRoot);
 
                     using (var context = renderTarget.CreateDrawingContext())
@@ -305,26 +305,6 @@ namespace Avalonia.Rendering
             return _overlay;
         }
 
-        private IRenderTargetBitmapImpl GetRenderTargetForLayer(Scene scene, IVisual layerRoot)
-        {
-            var size = new Size(scene.Size.Width * scene.Scaling, scene.Size.Height * scene.Scaling);
-            RenderLayer result;
-
-            if (_layers.TryGetValue(layerRoot, out result))
-            {
-                if (result.Size != scene.Size)
-                {
-                    result.ResizeBitmap(size, scene.Scaling);
-                }
-            }
-            else
-            {
-                _layers.Add(layerRoot, size, scene.Scaling);
-            }
-
-            return _layers[layerRoot].Bitmap;
-        }
-
         private void SaveDebugFrames(int id)
         {
             var index = 0;

src/Avalonia.Visuals/Rendering/RenderLayer.cs

@@ -16,7 +16,7 @@ namespace Avalonia.Rendering
             IVisual layerRoot)
         {
             _factory = factory;
-            Bitmap = factory.CreateLayer(layerRoot, size, 96 * scaling, 96 * scaling);
+            Bitmap = factory.CreateLayer(layerRoot, size * scaling, 96 * scaling, 96 * scaling);
             Size = size;
             Scaling = scaling;
             LayerRoot = layerRoot;

src/Avalonia.Visuals/Rendering/RenderLayers.cs

@@ -20,24 +20,25 @@ namespace Avalonia.Rendering
         public int Count => _inner.Count;
         public RenderLayer this[IVisual layerRoot] => _index[layerRoot];
 
-        public RenderLayer Add(IVisual layerRoot, Size size, double scaling)
+        public void Update(Scene scene)
         {
-            RenderLayer result;
-
-            if (!_index.TryGetValue(layerRoot, out result))
+            for (var i = scene.Layers.Count - 1; i >= 0; --i)
             {
-                result = new RenderLayer(_factory, size, scaling, layerRoot);
-                _inner.Add(result);
-                _index.Add(layerRoot, result);
-            }
+                var src = scene.Layers[i];
+                RenderLayer layer;
 
-            return result;
-        }
-
-        public bool Exists(IVisual layerRoot) => _index.ContainsKey(layerRoot);
+                if (!_index.TryGetValue(src.LayerRoot, out layer))
+                {
+                    layer = new RenderLayer(_factory, scene.Size, scene.Scaling, src.LayerRoot);
+                    _inner.Add(layer);
+                    _index.Add(src.LayerRoot, layer);
+                }
+                else
+                {
+                    layer.ResizeBitmap(scene.Size, scene.Scaling);
+                }
+            }
 
-        public void RemoveUnused(Scene scene)
-        {
             for (var i = _inner.Count - 1; i >= 0; --i)
             {
                 var layer = _inner[i];