'Image-transformation filter.
Does the same as VideoClip.fl_image, but for ImageClip the
tranformed clip is computed once and for all at the beginning,
and not for each \'frame\'.'

@outplace
def fl_image(self, image_func, apply_to=None):

    if (apply_to is None):
        apply_to = []
    arr = image_func(self.get_frame(0))
    self.size = arr.shape[:2][::(-1)]
    self.make_frame = (lambda t: arr)
    self.img = arr
    for attr in apply_to:
        if hasattr(self, attr):
            a = getattr(self, attr)
            if (a is not None):
                new_a = a.fl_image(image_func)
                setattr(self, attr, new_a)
