'returns a tuple (data, width, height), where:
- data is a string with raw bytes representing the pixels in 3-channel RGB
(i.e. every three bytes = 1 pixel)
- width is the width of the image
- height is the height of the image'

def get_image(self):

    (width, height) = self.get_dimensions()
    gl.glReadBuffer(gl.GL_BACK)
    data = gl.glReadPixels(0, 0, width, height, gl.GL_RGB, gl.GL_UNSIGNED_BYTE)
    return (data, width, height)
