ENH: By default, compute a lossless bitrate.

For `pims.play` and `pims.export`, the user can specify a bitrate. The current default, when `bitrate=None`, is a crude guess that does not scale with frame shape. This PR guarantees that the default bitrate will produce a lossless video for any frame shape or output frame rate.

If the user wants a lossy video, a smaller `bitrate` can of course be specified.