[CoreAnimation] Ensure that we increase the handle refenrece count in MakeMutable. Fixes #3089 (#3099)

* [CoreAnimation] Ensure that we increate the handle referene in MakeMutable. Fixes 3089

Fixes https://github.com/xamarin/xamarin-macios/issues/3089

src/CoreGraphics/CGPath.cs

@@ -560,9 +560,7 @@ namespace XamCore.CoreGraphics {
 		static CGPath MakeMutable (IntPtr source)
 		{
 			var mutable = CGPathCreateMutableCopy (source);
-			CGPathRelease (source);
-
-			return new CGPath (mutable, true);
+			return new CGPath (mutable, false);
 		}
 
 		[DllImport (Constants.CoreGraphicsLibrary)]

tests/monotouch-test/CoreGraphics/PathTest.cs

@@ -9,6 +9,7 @@
 
 using System;
 using System.Drawing;
+using System.Runtime.InteropServices;
 #if XAMCORE_2_0
 using Foundation;
 using CoreGraphics;
@@ -16,6 +17,7 @@ using CoreGraphics;
 using MonoTouch.CoreGraphics;
 using MonoTouch.Foundation;
 #endif
+using ObjCRuntime;
 using NUnit.Framework;
 
 #if XAMCORE_2_0
@@ -34,6 +36,9 @@ namespace MonoTouchFixtures.CoreGraphics {
 	[Preserve (AllMembers = true)]
 	public class PathTest {
 
+		[DllImport (Constants.CoreFoundationLibrary)]
+		extern static nint CFGetRetainCount (IntPtr handle);
+
 		[Test]
 		public void EllipseFromRect ()
 		{
@@ -259,5 +264,21 @@ namespace MonoTouchFixtures.CoreGraphics {
 				Assert.IsNotNull (path, "path");
 			}
 		}
+
+		[Test]
+		public void IncreaseRetainCountMakeMutable ()
+		{
+			// ensure that we do not crash and that the retain count is changed.
+			using (CGPath p1 = new CGPath ())
+			{
+				var count = CFGetRetainCount (p1.Handle);
+				using (var copy = p1.Copy ())
+				{
+					var newRetainCount = CFGetRetainCount (copy.Handle);
+					Assert.AreNotEqual (count, newRetainCount, $"Count is {newRetainCount} and was {count}");
+					Assert.IsTrue (count < newRetainCount);
+				}
+			}
+		}
 	}
 }