[Tile-serving] [openstreetmap/osm2pgsql] Handling of "Multi" geometries in flex output (#1316)

[mboeringa]

@joto 

A lot of sense in what you write, and I agree with most of it.

Just one remark about the potential splitting of Polygons: it would be most logical to use [ST_Subdivide](http://postgis.net/docs/ST_Subdivide.html) for this. Do note that this is a far more expensive operation than simply splitting LineStrings. Also note that splitting polygons comes with a couple of caveats:

- Any internal rings / holes that are being cut by the subdivision lines can no longer be removed during any subsequent generalization step, as they are no longer inner holes.
- Any smoothing step applied after the splitting, e.g. using [ST_ChaikinSmoothing](https://postgis.net/docs/ST_ChaikinSmoothing.html) will cause (small) gaps at the crossing of subdivision lines, as the corners will collapse inwards. 
- Subdivision lines can be very long and cause issues when attempting to reproject data or using on-the-fly reprojection. Adding in a configurable densification step using e.g. [ST_Segmentize](https://postgis.net/docs/ST_Segmentize.html) may be desirable in some cases.

Probably all a bit out of scope here, but it may be good to keep this in mind anyway.

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