Worry-free plotting

[paleolimbot]

Currently the plot function resolves everything to WKB and then plots using wkutils. wk's plotting is now just as good, so let's use it! Plus, we have a whole geometry engine at our disposal, so we can do simplification (at the sub-pixel level so that topology doesn't matter), crop, and intersection query to make this fast. You can still crash your R session (by trying to plot large numbers of tiny features), but that's a battle for another day (and perhaps a more comprehensive plotting library).