[RENDER] Don't copy "resized" layer contents + properly remove unneded layers

src/Avalonia.Visuals/Rendering/DeferredRenderer.cs

@@ -336,16 +336,34 @@ namespace Avalonia.Rendering
 
         private void RenderToLayers(Scene scene)
         {
-            if (scene.Layers.HasDirty)
+            foreach (var layer in scene.Layers)
             {
-                foreach (var layer in scene.Layers)
-                {
-                    var renderTarget = Layers[layer.LayerRoot].Bitmap;
-                    var node = (VisualNode)scene.FindNode(layer.LayerRoot);
+                var renderLayer = Layers[layer.LayerRoot];
+                if (layer.Dirty.IsEmpty && !renderLayer.IsEmpty)
+                    continue;
+                var renderTarget = renderLayer.Bitmap;
+                var node = (VisualNode)scene.FindNode(layer.LayerRoot);
 
-                    if (node != null)
+                if (node != null)
+                {
+                    using (var context = renderTarget.Item.CreateDrawingContext(this))
                     {
-                        using (var context = renderTarget.Item.CreateDrawingContext(this))
+                        if (renderLayer.IsEmpty)
+                        {
+                            // Render entire layer root node
+                            context.Clear(Colors.Transparent);
+                            context.Transform = Matrix.Identity;
+                            context.PushClip(node.ClipBounds);
+                            Render(context, node, layer.LayerRoot, node.ClipBounds);
+                            context.PopClip();
+                            if (DrawDirtyRects)
+                            {
+                                _dirtyRectsDisplay.Add(node.ClipBounds);
+                            }
+
+                            renderLayer.IsEmpty = false;
+                        }
+                        else
                         {
                             foreach (var rect in layer.Dirty)
                             {
@@ -364,6 +382,7 @@ namespace Avalonia.Rendering
                     }
                 }
             }
+
         }
 
         private void RenderOverlay(Scene scene, IDrawingContextImpl parentContent)

src/Avalonia.Visuals/Rendering/RenderLayer.cs

@@ -7,39 +7,39 @@ namespace Avalonia.Rendering
 {
     public class RenderLayer
     {
-        private readonly IDrawingContextImpl _drawingContext;
-
         public RenderLayer(
             IDrawingContextImpl drawingContext,
             Size size,
             double scaling,
             IVisual layerRoot)
         {
-            _drawingContext = drawingContext;
             Bitmap = RefCountable.Create(drawingContext.CreateLayer(size));
             Size = size;
             Scaling = scaling;
             LayerRoot = layerRoot;
+            IsEmpty = true;
         }
 
         public IRef<IRenderTargetBitmapImpl> Bitmap { get; private set; }
+        public bool IsEmpty { get; set; }
         public double Scaling { get; private set; }
         public Size Size { get; private set; }
         public IVisual LayerRoot { get; }
 
-        public void ResizeBitmap(Size size, double scaling)
+        public void RecreateBitmap(IDrawingContextImpl drawingContext, Size size, double scaling)
         {
             if (Size != size || Scaling != scaling)
             {
-                var resized = RefCountable.Create(_drawingContext.CreateLayer(size));
+                var resized = RefCountable.Create(drawingContext.CreateLayer(size));
 
                 using (var context = resized.Item.CreateDrawingContext(null))
                 {
                     context.Clear(Colors.Transparent);
-                    context.DrawImage(Bitmap, 1, new Rect(Size), new Rect(Size));
                     Bitmap.Dispose();
                     Bitmap = resized;
+                    Scaling = scaling;
                     Size = size;
+                    IsEmpty = true;
                 }
             }
         }

src/Avalonia.Visuals/Rendering/RenderLayers.cs

@@ -29,11 +29,11 @@ namespace Avalonia.Rendering
                 }
                 else
                 {
-                    layer.ResizeBitmap(scene.Size, scene.Scaling);
+                    layer.RecreateBitmap(context, scene.Size, scene.Scaling);
                 }
             }
 
-            for (var i = _inner.Count - 1; i >= 0; --i)
+            for (var i = 0; i < _inner.Count;)
             {
                 var layer = _inner[i];
 
@@ -43,6 +43,8 @@ namespace Avalonia.Rendering
                     _inner.RemoveAt(i);
                     _index.Remove(layer.LayerRoot);
                 }
+                else
+                    i++;
             }
         }