Animated WebP / AVIF & Movies (AV1 / MP4) export in agg

ScreenToGif support them except AVIF via FFmpeg.
They’re newer and more efficient than GIF and have more-than-256-colors support.
I hope agg supports them.

All browsers have support for them, so we don’t need to care about the compatibility.

Hey @tats-u :wave:

I think a reasonable way to achieve this is something like I described here:

What do you think?

ScreenToGif uses FFmpeg. It’s fine that asciinema uses FFmpeg too.

For WebP, we might as well directly use libwebp, but much easier to depend on FFmpeg.