In general the player assumes that it won't receive out of order video.
This clearly can happen with separate L/R sources. A pass in L might
emit two frames which means the arrivals can't possibly be in order.
This commit fixes this by introducing a Shuffler which all alternate-3D
sources send their video to. The Shuffler re-orders things before they
arrive at the player.
It also fixes the code which inserts video frames before one that arrives
after a gap. This didn't cope with 3D right before.
The audio code solves a similar (perhaps the same?) problem with the
AudioMerger; perhaps we should have a similar thing for video and make
the player emit complete 3D frames.
Should help with #976.