MNT: Make sure overlays with a name starting with an underscore are shown in plot legends

In recent versions of matplotlib, the get_legend_handles_labels function filters out artists with a label beginning with a _ - see https://github.com/matplotlib/matplotlib/issues/21295 for more details.

This MR also contains a small bug fix to visualisation of rgb images - pre-filtered spline interpolation is now applied separately to each RGB channel, and the data is cast to float32 for the pre-filtering.