Fall back to indexing a Point in some cases

Index a Point instead of a low-coordinate-count Polygon, which is an indication that our geo manipulations may have gone overboard. This is a stopgap until we improve the detection and resolution of invalid geometries and the simplification of high-coordinate-count geometries.

/cc @hampelm 

Done. That's actually separate code for calculating the centroid. It had a bug for straight-line LineStrings, which we will start to encounter more thanks to sidewalk surveys.