geo + tip: true

[Fil]

This changes the derivation of a tip mark when there is only a geometry channel (typically, the geo mark) by:

• fixing a collapsing bug (Plot.geo with tip: true collapses the chart #1742)
• computing a centroid (unless there already is a channel for x or y) (Plot.geo with tip: true should use Plot.centroid #1743)

Demo: https://observablehq.com/@fil/geo-tip-true-1744

fixes #1742
closes #1743

cc: @yurivish

[Fil]

closing, I'm going to separate the bug fix and the proposed new feature