Finally probably achieved one of the biggest challenges so far...

Game main class:

        ShaderProgram.pedantic = false;

        mask = new Texture(Gdx.files.internal("bigSun1024violet.png"));
        viewMask = new Texture(Gdx.files.internal("bigSun256violet.png"));

        maskBuff = new FrameBuffer(Pixmap.Format.RGBA8888, frmSizeX, frmSizeY, false);
        msk = new TextureRegion(maskBuff.getColorBufferTexture());

        frmBuff = new FrameBuffer(Pixmap.Format.RGBA8888, frmSizeX, frmSizeY, false);
        frm = new TextureRegion(frmBuff.getColorBufferTexture());

        coverSprite = new Sprite(new Texture("violetCover.png"));
        float donutX = 0;
        float donutY = 0;
        coverSprite.setPosition(donutX, donutY);
        cam.update();

//        shader = new ShaderProgram(VERT, FRAG);
        shader = new ShaderProgram(Gdx.files.internal("vertex.glsl"), Gdx.files.internal("fragment-video.glsl"));
        if (!shader.isCompiled()) {
            System.err.println(shader.getLog());
            System.exit(0);
        }
        if (!shader.getLog().isEmpty())
            System.out.println(shader.getLog());

        shader.bind();
        shader.setUniformi("u_texture1", 1);
        shader.setUniformi("u_mask", 2);
        defaultShader = spriteBatch.getShader();

        //bind mask to glActiveTexture(GL_TEXTURE2)
        msk.getTexture().bind(2);

        //bind dirt to glActiveTexture(GL_TEXTURE1)
//      tex1.bind(1);
        frm.getTexture().bind(1);

        //now we need to reset glActiveTexture to zero!!!! since sprite batch does not do this for us
        Gdx.gl.glActiveTexture(GL20.GL_TEXTURE0);

Update class:

        arrowGame.lightX--;

        arrowGame.maskBuff.begin();
        Gdx.gl.glClearColor(255, 255, 255, 255);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        arrowGame.spriteBatch.begin();
        arrowGame.spriteBatch.draw(arrowGame.mask, arrowGame.lightX, 0, 0, 0, arrowGame.frmSizeX, arrowGame.frmSizeY, 1, 1, 0, 0, 0, arrowGame.frmSizeX, arrowGame.frmSizeY, false, true);
        arrowGame.spriteBatch.draw(arrowGame.viewMask, 500, 500, 0, 0, 256, 256, 1, 1, 0, 0, 0, 256, 256, false, true);
        arrowGame.spriteBatch.end();
        arrowGame.maskBuff.end();

Drawing class:

                    sb.setShader(arrowGame.shader);
                    arrowGame.coverSprite.draw(sb);
                    sb.setShader(arrowGame.defaultShader);