Convert to GIF in the background

[jschementi]

Currently the GIF maker converts the entire video to a GIF, even though we only export max-3 seconds. While this works for shorter videos, long videos can take very long to convert, and we have to do it at a poor quality.

Instead, the GIF maker should work against the video itself for seeking and such, but in the background convert it to a GIF at a good quality. When the conversion is complete, it should swap the media out so it plays back the GIF.

Playback modes like rebound/reverse won't work against the video, but that's OK.

When swapping the video and GIF, playback should be seamless, and not restart.

(Having this setup this way will allow us to just use the trimmer for videos too!)