geom::simplify is broken causing broken polygons at low zoom

[TimSC]

geom::simplify is glitchy and sometimes breaks polygons.

For alternatives, just using the Ramer–Douglas–Peucker algorithm on the inner and outer loops can break polygons. However, we could try feeding the simplified loops back into clipper the library to tidy them up.

Alternatively, we could introduce another library dependency such as https://trac.osgeo.org/geos/

UPDATE: can't reproduce this on master? is this just a big in scaleup3?

UPDATE 2: It does occur on master, but only seen with the TidalWater4326.shp sea shapes, which I suspect are more complex (including concave shapes?).

[TimSC]

Proposed fix using clipper a3911a5fcca6dfb9f7d68ac45bbb21c1db6f0bc0