Custom .af video format for WebCodecs: frame-accurate playback without <video>

> How does the file size of the custom .af container compare to a heavily optimized standard .mp4 or .webm?

mp4 / webm containers can be complex and "demuxing" those can be an expensive operation – that's why for my usage I decided to precompute certain things into a custom .af format

.af format consists in:

- raw frame data

- json manifest (number of frame, start/end byte for each frame, fps, etc.)

- last 4 byte indicating where the json manifest lives