Fix flickering movie when using embed_opencv mode

[EricThomson]

When movie shown in embed_opencv mode (currently in dev), it flickers. I'm not sure of the fix.

We discussed it some in the PR #1080 and I posted a movie of the phenomenon there.

One place to look for a potential solution: https://stackoverflow.com/questions/21360361/how-to-dynamically-update-a-plot-in-a-loop-in-ipython-notebook-within-one-cell/29675706#29675706