Fix joint video generation

When running the render_args=record and the metric animation has a different size than the rendered video (e.g. from mujoco), then no joint video is created. Here is the error i get when running simply python src/main.py render=record

/home/eppe/Scilab-RL/src/utils/animation_util.py:77: UserWarning: Attempting to set identical low and high ylims makes transformation singular; automatically expanding.
  ax_i.set_ylim(min(self.y_data[i]) - y_range * 0.05, max(self.y_data[i]) + y_range * 0.05)
[2023-11-13 17:55:49,641][matplotlib.animation][INFO] - Animation.save using <class 'matplotlib.animation.FFMpegWriter'>
[2023-11-13 17:55:49,642][matplotlib.animation][INFO] - MovieWriter._run: running command: ffmpeg -f rawvideo -vcodec rawvideo -s 500x500 -pix_fmt rgba -framerate 30 -loglevel error -i pipe: -vcodec h264 -pix_fmt yuv420p -y /home/eppe/Scilab-RL/data/ae141cc/FetchReach-v2/17-51-45/videos/eval-episode-40.metric.mp4
[Parsed_hstack_0 @ 0x5618459a5980] Input 1 height 500 does not match input 0 height 480.
[Parsed_hstack_0 @ 0x5618459a5980] Failed to configure output pad on Parsed_hstack_0
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #1:0

Fix this by padding the smaller video using e.g. this command: https://stackoverflow.com/questions/48485799/ffmpeg-extend-not-resize-video-size-by-adding-box-or-border