Make some rendering utils internal.

src/Avalonia.Base/Rendering/Utilities/TileBrushCalculator.cs

@@ -2,7 +2,7 @@
 
 namespace Avalonia.Rendering.Utilities
 {
-    public class TileBrushCalculator
+    internal class TileBrushCalculator
     {
         private readonly Size _imageSize;
         private readonly Rect _drawRect;

src/Avalonia.Base/Rendering/ZIndexComparer.cs

@@ -1,10 +1,9 @@
 using System;
 using System.Collections.Generic;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Rendering
 {
-    public class ZIndexComparer : IComparer<Visual>
+    internal class ZIndexComparer : IComparer<Visual>
     {
         public static readonly ZIndexComparer Instance = new ZIndexComparer();
         public static readonly Comparison<Visual> ComparisonInstance = Instance.Compare;