Fix update textures in handleContextRestored

* you try to iterate on PIXI.TextureCache as Array but it's Object;
* PIXI.WebGLRenderer.updateTexture deals with base texture, so we need to get such a field;
* need to texture._glTexture = null, because PIXI.WebGLRenderer.updateTexture only deal with texture without _glTexture;
* and also run as static method PIXI.WebGLRenderer.updateTexture(...);
Merge pull request #161 from Hyzhak/patch-1

Fix update textures in handleContextRestored