Allow GC to collect reused framebuffer, that's better than AccessViolationException from render thread

src/Windows/Avalonia.Win32/FramebufferManager.cs

@@ -5,7 +5,7 @@ using Avalonia.Win32.Interop;
 
 namespace Avalonia.Win32
 {
-    class FramebufferManager : IFramebufferPlatformSurface, IDisposable
+    class FramebufferManager : IFramebufferPlatformSurface
     {
         private readonly IntPtr _hwnd;
         private WindowFramebuffer _fb;
@@ -29,10 +29,5 @@ namespace Avalonia.Win32
             }
             return _fb;
         }
-
-        public void Dispose()
-        {
-            _fb?.Deallocate();
-        }
     }
 }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -13,6 +13,7 @@ using Avalonia.Input.Raw;
 using Avalonia.OpenGL;
 using Avalonia.Platform;
 using Avalonia.Rendering;
+using Avalonia.Threading;
 using Avalonia.Win32.Input;
 using Avalonia.Win32.Interop;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
@@ -234,8 +235,6 @@ namespace Avalonia.Win32
 
         public void Dispose()
         {
-            _framebuffer?.Dispose();
-            _framebuffer = null;
             if (_hwnd != IntPtr.Zero)
             {
                 UnmanagedMethods.DestroyWindow(_hwnd);