Portrayal : remove final and extend MapItem on MapLayer class

git-svn-id: https://svn.apache.org/repos/asf/sis/trunk@1833333 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/core/sis-portrayal/src/main/java/org/apache/sis/internal/map/MapLayer.java b/core/sis-portrayal/src/main/java/org/apache/sis/internal/map/MapLayer.java
index f255b89..692db42 100644
--- a/core/sis-portrayal/src/main/java/org/apache/sis/internal/map/MapLayer.java
+++ b/core/sis-portrayal/src/main/java/org/apache/sis/internal/map/MapLayer.java
@@ -38,7 +38,7 @@
  * @since   1.0
  * @module
  */
-public final class MapLayer {
+public class MapLayer extends MapItem {
     /**
      * Data to be rendered.
      */