Video source repaint after seeked

## Motivation

I need the ability to set or update the currentTime of a video in mapbox-gl while the video is paused. As the video source is written now the video source is only painted while the video is playing, so after pausing and changing the time, no visible change is seen in mapbox-gl.

## Design Alternatives

You could add another event listener to the video for the "seeked" event and when that event fires, set a flag to repaint the video which is then removed after being painted once.

## Design

Potential issues: The hasTransition() and prepare() functions might not always execute in the correct order due to a race condition, one additional flag may be needed to ensure that prepare() is run first, then hasTransition()

### Mock-Up

Potential solution, see patch attached to this request or the demo [here](https://dwilson2547.github.io/mapbox-video-controls-example/index.html)

### Concepts

This shouldn't affect the existing functionality of the video source, I'm hoping to add to it in such a way that no one has to modify existing code using the video source.

### Implementation

[videofix.txt](https://github.com/mapbox/mapbox-gl-js/files/2631392/videofix.txt)

Review the attached videofix.txt and the video source implementation, then reproduce the paused-video seek case using the linked demo. Trace how video playback triggers repainting and determine where the seeked event fits with prepare() and hasTransition(). Done means changing currentTime while paused visibly updates the map without changing existing video-source behavior.

Written by the indexing model from the issue text.