Improve MultiPolygon centroid robustness

Do this by allowing each polygon shell to set its own origin point,
instead of re-using the origin point from the first polygon in
the geometry.

modules/core/src/main/java/org/locationtech/jts/algorithm/Centroid.java

@@ -153,7 +153,6 @@ else if (ptCount > 0){
 
   private void setBasePoint(Coordinate basePt)
   {
-    if (this.areaBasePt == null)
       this.areaBasePt = basePt;
   }