Add video as a movable object

[TheShadow29]

Description of proposed feature

Currently, only Image is available as a movable object using ImageMobject. The idea is to allow VideoMobject.

This would allow importing a video that was not created via Manim super-easy.

How can the new feature be used?

I see two ways it can be used

1. The entire video is played first, then other manim operations are resumed.
2. Both video and manim operations happen concurrently.

The first one should be relatively straightforward (I think?) by repeatedly calling ImageMobject with updated frames. The second one would require synchronizing the frame-rate of the video with the manim operations.

[kolibril13]

There was a short discussion about it here #260. I've also tried to implement that once here https://github.com/ManimCommunity/manim/pull/129/files , but did not find a good way to do this and did not follow up on that. When you want to work on this, that would be a great addition to manim!

[TheShadow29]

@kolibril13 Ah, I did search for video, but not specifically for VideoMobject so I missed your pr.

It seems #129 discusses similar issues as noted here. I can take a stab at it sometime this week.

At present, the easiest way to go about it seems to be using pyav or imageio (with imageio-ffmpeg plugin) and generating the image files from the video at run-time.

What do you think?