render stencil portals (transparent)

Assets/Portal/Runtime/Portal.cs

@@ -26,9 +26,10 @@ namespace EscapeRoomEngine.Portal.Runtime
         /// </summary>
         public float minNearClipPlane = 0.0001f;
         /// <summary>
-        /// The mesh where the portal will be drawn.
+        /// The meshes where the portal will draw its stencil and depth.
         /// </summary>
-        [BoxGroup("Internal")] public MeshRenderer screen;
+        [BoxGroup("Internal")] [SerializeField]
+        private MeshRenderer screen;
         /// <summary>
         /// The transform marking the edge of the portal plane.
         /// </summary>