raster images not updating when set_channel on matched secondary images

[kswang1029]

Test branch: #123
Example script:

    session = test_session_initializer()
    session.overlay.set_view_area(800, 400)
    imgs = session.open_images(["HD163296_CO_2_1_subimage.fits", 
                                "HD163296_13CO_2-1_subimage.fits"])
    imgs[1].set_cube_matching(True)
    imgs[0].set_channel(88)
    #imgs[1].set_channel(60)
    session.save_rendered_view("raster_rendering.png")

With the example above we will see the png image as

However, if we apply set_channel to imgs[1] (matched image) instead, we will see

We would expect to see identical png images when we apply set_channel(88) to imgs[0] and set_channel(60) to imgs[1]