Brain.save_movie() II

Alternative to https://github.com/nipy/PySurfer/pull/79 co-opting `Brain.make_montage()`

If that warning were not an issue, what would the ideal API be? This shouldn't be a problem if we eventually move to a different plotting backend (e.g., `vispy`), so I'd hate to compromise the API for the sake of this potential issue. I had to put that warning in to be safe, but I don't think complications arising from it are too prevalent.