[Mac] CursorVisible should only apply when changed

This fixes an issue where `NativeWindow.Dispose()` would move the mouse
cursor.

diff --git a/Source/OpenTK/Platform/MacOS/CocoaNativeWindow.cs b/Source/OpenTK/Platform/MacOS/CocoaNativeWindow.cs
index d4d6b48..db0e314 100644 (file)
--- a/Source/OpenTK/Platform/MacOS/CocoaNativeWindow.cs
+++ b/Source/OpenTK/Platform/MacOS/CocoaNativeWindow.cs
@@ -1000,15 +1000,15 @@ namespace OpenTK.Platform.MacOS
             get { return cursorVisible; }
             set
             {
-                cursorVisible = value;
-                if (value)
+                if (value && !cursorVisible)
                 {
                     SetCursorVisible(true);
                 }
-                else
+                else if (!value && cursorVisible)
                 {
                     SetCursorVisible(false);
                 }
+                cursorVisible = value;
             }
         }