Add pre-render method to VRViewManager in preparation for adding Oculus

Rift support. See
https://hub.jmonkeyengine.org/t/libovr-oculus-rift-support/39427

jme3-vr/src/main/java/com/jme3/app/VRAppState.java

@@ -432,6 +432,16 @@ public class VRAppState extends AbstractAppState {
         }
     }
 
+    @Override
+    public void render(RenderManager rm) {
+        super.render(rm);
+
+        // update compositor
+        if( environment.getVRViewManager() != null ) {
+            environment.getVRViewManager().render();
+        }
+    }
+
     @Override
     public void initialize(AppStateManager stateManager, Application app) {
         super.initialize(stateManager, app);

jme3-vr/src/main/java/com/jme3/util/AbstractVRViewManager.java

@@ -128,6 +128,10 @@ public abstract class AbstractVRViewManager implements VRViewManager {
     public VREnvironment getVREnvironment(){
     	return environment;
     }
+
+    @Override
+    public void render() {
+    }
     
     /**
      * Handles moving filters from the main view to each eye

jme3-vr/src/main/java/com/jme3/util/VRViewManager.java

@@ -150,6 +150,12 @@ public interface VRViewManager {
      */
     public void update(float tpf);
     
+    /**
+     * Set up the scene for rendering.
+     * This method should be called before any rendering takes place.
+     */
+    public void render();
+
     /**
      * Send the rendering result as textures to the two eyes. 
      * This method should be called after all the rendering operations