Reduce the scope of NGShape3D's members and methods

Most of the protected members in NGShape3D should be changed to private or package scope. Some of the methods should also be changed too. While making this change the following clean up should also be done:

1) Remove unused method
    protected NGTriangleMesh getMesh() {
        return mesh;
    }

2) Added @override to
    protected void renderContent(Graphics g) {