Added a protected modifier to getReceivers in Abstract shadow renderer

see issue https://github.com/jMonkeyEngine/jmonkeyengine/issues/212

jme3-core/src/main/java/com/jme3/shadow/AbstractShadowRenderer.java

@@ -459,7 +459,7 @@ public abstract class AbstractShadowRenderer implements SceneProcessor, Savable
         debug = true;
     }
 
-    abstract void getReceivers(GeometryList lightReceivers);
+    protected abstract void getReceivers(GeometryList lightReceivers);
 
     public void postFrame(FrameBuffer out) {
         if (skipPostPass) {