Added onBindObservable on effect

src/Materials/babylon.effect.ts

@@ -93,6 +93,7 @@
         public uniqueId = 0;
         public onCompileObservable = new Observable<Effect>();
         public onErrorObservable = new Observable<Effect>();
+        public onBindObservable = new Observable<Effect>();
 
         private static _uniqueIdSeed = 0;
         private _engine: Engine;

src/babylon.engine.ts

@@ -2029,6 +2029,7 @@
             if (effect.onBind) {
                 effect.onBind(effect);
             }
+            effect.onBindObservable.notifyObservers(effect);
         }
 
         public setIntArray(uniform: WebGLUniformLocation, array: Int32Array): void {