Optical Flow (show_pred): Make a video instead of showing the results in a window

[v-iashin]

Currently, if show_pred argument is used with a optical flow feature extractor, the window dialog pops-up where a user needs to press some button.

Problem: it is nice but, when a machine does not have an attached screen, it fails.

Idea: we could avoid it by making an mp4 video with RGB and optical flow (or a .gif).

Implementation: maybe with cv2, maybe with imageio.get_writer

RFC: should we keep the old behaviour as an argument? Is it useful for anyone or this is just me enjoying looking at them manually?