Mark unstable math utilities (#15278)

* Mark MathUtilities unstable

* Mark new methods from 11.1 as internal in MathUtilities

* Fix for build task

---------

Co-authored-by: Benedikt Stebner <Gillibald@users.noreply.github.com>

src/Avalonia.Base/Utilities/MathUtilities.cs

@@ -1,5 +1,8 @@
 using System;
 using System.Runtime.CompilerServices;
+#if !BUILDTASK
+using Avalonia.Metadata;
+#endif
 
 namespace Avalonia.Utilities
 {
@@ -7,6 +10,7 @@ namespace Avalonia.Utilities
     /// Provides math utilities not provided in System.Math.
     /// </summary>
 #if !BUILDTASK
+    [Unstable("This API might be removed in next major version. Please use corresponding BCL APIs.")]
     public
 #endif
     static class MathUtilities
@@ -361,7 +365,7 @@ namespace Avalonia.Utilities
         }
         
 #if !BUILDTASK
-        public static int WhichPolygonSideIntersects(
+        internal static int WhichPolygonSideIntersects(
             uint cPoly,
             ReadOnlySpan<Vector> pPtPoly,
             Vector ptCurrent,
@@ -400,7 +404,7 @@ namespace Avalonia.Utilities
             return nPositive > 0 ? 1 : -1;
         }
 
-        public static bool DoPolygonsIntersect(
+        internal static bool DoPolygonsIntersect(
             uint cPolyA,
             ReadOnlySpan<Vector> pPtPolyA,
             uint cPolyB,
@@ -427,7 +431,7 @@ namespace Avalonia.Utilities
             return true;
         }
         
-        public static bool IsEntirelyContained(
+        internal static bool IsEntirelyContained(
             uint cPolyA,
             ReadOnlySpan<Vector> pPtPolyA,
             uint cPolyB,