Dispose window framebuffers.

src/Shared/PlatformSupport/StandardRuntimePlatform.cs

@@ -86,11 +86,15 @@ namespace Avalonia.Shared.PlatformSupport
 #if DEBUG
                 if (Thread.CurrentThread.ManagedThreadId == GCThread?.ManagedThreadId)
                 {
-                    lock(_lock)
+                    lock (_lock)
+                    {
                         if (!IsDisposed)
+                        {
                             Console.Error.WriteLine("Native blob disposal from finalizer thread\nBacktrace: "
-                                                    + Environment.StackTrace
-                                                    + "\n\nBlob created by " + _backtrace);
+                                                 + Environment.StackTrace
+                                                 + "\n\nBlob created by " + _backtrace);
+                        }
+                    }
                 }
 #endif
                 DoDispose();

src/Windows/Avalonia.Win32/FramebufferManager.cs

@@ -5,7 +5,7 @@ using Avalonia.Win32.Interop;
 
 namespace Avalonia.Win32
 {
-    class FramebufferManager : IFramebufferPlatformSurface
+    class FramebufferManager : IFramebufferPlatformSurface, IDisposable
     {
         private readonly IntPtr _hwnd;
         private WindowFramebuffer _fb;
@@ -31,5 +31,11 @@ namespace Avalonia.Win32
 
             return _fb;
         }
+
+        public void Dispose()
+        {
+            _fb?.Deallocate();
+            _fb = null;
+        }
     }
 }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -389,6 +389,8 @@ namespace Avalonia.Win32
                 UnregisterClass(_className, GetModuleHandle(null));
                 _className = null;
             }
+
+            _framebuffer.Dispose();
         }
 
         public void Invalidate(Rect rect)