fix mp4 encoding error

[hkunzhe]

As discussed in #377, the latest pyav package (version 12.0.2) doesn't support the libx264 encoder. Since L156 will install imageio[pyav], the error can not be reproduced if you saved the result in the GIF format before.

A Minimal, Reproducible Example:

# conda create -n av_test python=3.10
# conda activate av_test && pip install pyav

import av

print('h264' in av.codecs_available). # False

with av.open("test.mp4", mode="w") as container:
    stream = container.add_stream('libx264', rate=60)
# av.codec.codec.UnknownCodecError: libx264

Install imageio[av] will fix it.

# conda create -n av_test_1 python=3.10
# conda activate av_test_1 && pip install imageio[av]

import av

print('h264' in av.codecs_available)

with av.open("test.mp4", mode="w") as container:
    stream = container.add_stream('libx264', rate=60)