GIF logic unimplemented on v4

[KTibow]

pufferl.py has all this logic:

    parser.add_argument('--render-mode', type=str, default='auto',
        choices=['auto', 'human', 'ansi', 'rgb_array', 'raylib', 'None'])
# ...
    parser.add_argument('--save-frames', type=int, default=0)
    parser.add_argument('--gif-path', type=str, default='eval.gif')
    parser.add_argument('--fps', type=float, default=15)

that isn't actually tied up to anything. It was broken in 26f2f5a. Bring it back (preferred, this would be nice on headless machines!) or drop it.