Add caching layer for video frames

[jasonjmcghee]

When scrolling through the timeline, it's very expensive to keep opening videos and pulling out frames.

We need to pull all frames for a video chunk into memory and make them available via a cache.

It would be better to load the "next chunk" for the timeline as well to keep in front of the user so there's no waiting time.

Ideally we'd keep around a bunch of frames in the cache so the user could scroll back and forth and it is instant / snappy.