Make instant inspection respect spreading

[droumis]

Issue originally authored by @jbednar:

Using https://examples.holoviz.org/ship_traffic , instant inspection seemed to work when hovering over dense areas, but once I zoomed in, I was nearly unable to hit any isolated points (just a very occasional flash of a change in the hover. I modified the file to have much bigger points from more spreading (pointsp = hd.dynspread(hd.datashade(pts2, color_key=color_key, aggregator=ds.count_cat('category'), min_alpha=90), max_px=7, threshold=0.9)), and had no more luck. It looks to me like the instant inspection information is not being spread the way the visible points are.

[jbednar]

Subsequent discussions with @jlstevens made it clear that the demos where this was working depended on the hackier version of categorical instant inspection, with a separately overlaid hit-testing layer on top of a datashader-rendered categorical plot. Here we may have to do some thinking about how to spread the index or data layers according to the same rules used on the plot layers.