Layers: add contains methods (#1032)

vtm/src/org/oscim/map/Layers.java

@@ -209,6 +209,20 @@ public final class Layers extends AbstractList<Layer> {
         mGroupIndex.put(group, mLayerList.size());
     }
 
+    /**
+     * @see List#contains(Object)
+     */
+    public synchronized boolean contains(Layer layer) {
+        return mLayerList.contains(layer);
+    }
+
+    /**
+     * @see List#contains(Object)
+     */
+    public synchronized boolean containsGroup(int group) {
+        return mGroupList.contains(group);
+    }
+
     /**
      * Should only be used by MapRenderer.
      *