Resetting the native shader instance when the shader changes.

     * @return       shader

     */

    public Shader setShader(Shader shader) {

        // If mShader changes, cached value of native shader aren't valid, since

        // old shader's pointer may be reused by another shader allocation later

        if (mShader != shader) {

            mNativeShader = -1;

        }

        // Defer setting the shader natively until getNativeInstance() is called

        mShader = shader;

        return shader;