`ConcaveHullOfPolygons` can produce disconnected result

[dr-jts]

ConcaveHullOfPolygons can produce a disconnected MultiPolygon output in some situations. This is contrary to the stated contract for this class.

This is due to a design limitation in the current code. It can be fixed by ensuring that triangle removal does not disconnect the triangulation.

Notes

• It might be nice to provide an option to allow disconnected output, if this is desired (also for ConcaveHull for points).

Example

MULTIPOLYGON (((10 20, 20 20, 20 10, 10 10, 10 20)), ((20 30, 30 30, 30 20, 20 20, 20 30)), ((80 90, 90 90, 90 80, 80 80, 80 90)))

Result for Length Ratio = 0.1

MULTIPOLYGON (((20 30, 30 30, 30 20, 20 10, 10 10, 10 20, 20 30)), ((90 90, 90 80, 80 80, 80 90, 90 90)))