Add drift mode in wav, rm, dir, and speed plots

[d1manson]

With the latest rewrite, even wav-drift shouldn't be too hard: in density mode we currently use the red channel for counting, but we can use the green channel for summing total times, and then do the division in the second stage of the render.

Not sure exactly what plots to show for the other things.

For rm, might use brightness for rate and red-green for some kind of weighted mean time, with the weighting used to ensure we do normalise by dwell somehow.

For speed and dir, we are currently showing a 1d dataset, so adding a second dimension isn't hard, but has do be done in a useful way.