Change private property to protected

for make class inherited from standard material i need access to private property 
so this is acceptable changes for core

src/Materials/babylon.standardMaterial.ts

@@ -1,5 +1,5 @@
-module BABYLON {
-    class StandardMaterialDefines extends MaterialDefines {
+module BABYLON {
+   export class StandardMaterialDefines extends MaterialDefines {
         public DIFFUSE = false;
         public AMBIENT = false;
         public OPACITY = false;
@@ -24,7 +24,6 @@
         public EMISSIVEFRESNEL = false;
         public FRESNEL = false;
         public NORMAL = false;
-        public TANGENT = false;
         public UV1 = false;
         public UV2 = false;
         public VERTEXCOLOR = false;
@@ -199,15 +198,15 @@
         @serializeAsColorCurves()
         public cameraColorCurves: ColorCurves = null;
 
-        private _renderTargets = new SmartArray<RenderTargetTexture>(16);
-        private _worldViewProjectionMatrix = Matrix.Zero();
-        private _globalAmbientColor = new Color3(0, 0, 0);
-        private _renderId: number;
+        protected _renderTargets = new SmartArray<RenderTargetTexture>(16);
+        protected _worldViewProjectionMatrix = Matrix.Zero();
+        protected _globalAmbientColor = new Color3(0, 0, 0);
+        protected _renderId: number;
 
-        private _defines = new StandardMaterialDefines();
-        private _cachedDefines = new StandardMaterialDefines();
+        protected _defines = new StandardMaterialDefines();
+        protected _cachedDefines = new StandardMaterialDefines();
 
-        private _useLogarithmicDepth: boolean;
+        protected _useLogarithmicDepth: boolean;
 
         constructor(name: string, scene: Scene) {
             super(name, scene);
@@ -250,7 +249,7 @@
             return this.diffuseTexture != null && this.diffuseTexture.hasAlpha;
         }
 
-        private _shouldUseAlphaFromDiffuseTexture(): boolean {
+        protected _shouldUseAlphaFromDiffuseTexture(): boolean {
             return this.diffuseTexture != null && this.diffuseTexture.hasAlpha && this.useAlphaFromDiffuseTexture;
         }
 
@@ -259,7 +258,7 @@
         }
 
         // Methods   
-        private _checkCache(scene: Scene, mesh?: AbstractMesh, useInstances?: boolean): boolean {
+        protected _checkCache(scene: Scene, mesh?: AbstractMesh, useInstances?: boolean): boolean {
             if (!mesh) {
                 return true;
             }
@@ -548,9 +547,6 @@
             if (mesh) {
                 if (needNormals && mesh.isVerticesDataPresent(VertexBuffer.NormalKind)) {
                     this._defines.NORMAL = true;
-                    if (mesh.isVerticesDataPresent(VertexBuffer.TangentKind)) {
-                        this._defines.TANGENT = true;
-                    }
                 }
                 if (needUVs) {
                     if (mesh.isVerticesDataPresent(VertexBuffer.UVKind)) {
@@ -655,10 +651,6 @@
                     attribs.push(VertexBuffer.NormalKind);
                 }
 
-                if (this._defines.TANGENT) {
-                    attribs.push(VertexBuffer.TangentKind);
-                }
-
                 if (this._defines.UV1) {
                     attribs.push(VertexBuffer.UVKind);
                 }
@@ -995,8 +987,6 @@
                 }
             }
 
-            this._renderTargets.dispose();
-
             super.dispose(forceDisposeEffect, forceDisposeTextures);
         }