Update VideoFrame ctor to take single buffer

[chcunningham]

The original "planes" constructor was designed for coherence with the planes API. But Planes is now replaced by frame.copyTo(destination, ...) (as of #208), which copies the entire frame copy to a single buffer. Its natural to also update the Planes constructor to match that style, taking in a single buffer and an optional sequence of PlaneLayouts. PR comments raised the issue here.