Make InvalidateMirrorTransform protected.

src/Avalonia.Base/Visual.cs

@@ -774,7 +774,7 @@ namespace Avalonia
         /// Computes the <see cref="HasMirrorTransform"/> value according to the 
         /// <see cref="FlowDirection"/> and <see cref="BypassFlowDirectionPolicies"/>
         /// </summary>
-        public virtual void InvalidateMirrorTransform()
+        protected internal virtual void InvalidateMirrorTransform()
         {
             var flowDirection = this.FlowDirection;
             var parentFlowDirection = FlowDirection.LeftToRight;

src/Avalonia.Controls/ComboBox.cs

@@ -164,7 +164,7 @@ namespace Avalonia.Controls
             UpdateSelectionBoxItem(SelectedItem);
         }
 
-        public override void InvalidateMirrorTransform()
+        protected internal override void InvalidateMirrorTransform()
         {
             base.InvalidateMirrorTransform();
             UpdateFlowDirection();

src/Avalonia.Controls/TopLevel.cs

@@ -740,7 +740,7 @@ namespace Avalonia.Controls
 
         protected override bool BypassFlowDirectionPolicies => true;
 
-        public override void InvalidateMirrorTransform()
+        protected internal override void InvalidateMirrorTransform()
         {
             // Do nothing becuase TopLevel should't apply MirrorTransform on himself.
         }