Hello spear_kor,
Which error is gstreamer mentioned? Please gst-inspect mfw_v4lsink in order to see how to control display size.
You can add the glRectf as example:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(0.0f, 0.0f, 0.0f);
glRectf(-0.75f,0.75f, 0.75f, -0.75f);
glutSwapBuffers();Regards