Pause media recorder when waiting to load during playback

[clabe45]

When recording a movie with Video or Audio layers, it is possible for the <video> and <audio> elements to buffer during playback. The movie currently waits to start recording until _whenReady resolves, which prevents buffering in the beginning of the recording but not in the middle. If the movie's ready property becomes false during playback, _render keeps noop-looping until it is ready. We need to pause the MediaRecorder object that record uses when this occurs.

Relevant Movie methods:

• _whenReady resolves when all its layers' and effects' whenReady methods resolve (i.e. when Movie#ready is true)
• _render renders every layer, and repeats the next frame using requestAnimationFrame
• record calls _render after the movie is ready