Make Renderer.initialize() a standard method since everyone already uses it anyway

jme3-core/src/main/java/com/jme3/renderer/Renderer.java

@@ -55,6 +55,8 @@ import java.util.EnumSet;
  */
 public interface Renderer {
 
+    public void initialize();
+    
     /**
      * Get the capabilities of the renderer.
      * @return The capabilities of the renderer.

jme3-core/src/main/java/com/jme3/system/NullRenderer.java

@@ -54,6 +54,9 @@ public class NullRenderer implements Renderer {
     private static final EnumSet<Caps> caps = EnumSet.noneOf(Caps.class);
     private static final Statistics stats = new Statistics();
 
+    public void initialize() {
+    }
+    
     public EnumSet<Caps> getCaps() {
         return caps;
     }