CPL_area(): use proper casts to OGRGeometry subtypes, in particular t…

…o fix issues with GeometryCollection of Polygon

Fixes #2466

src/gdal_geom.cpp

@@ -12,13 +12,16 @@ Rcpp::NumericVector CPL_area(Rcpp::List sfc) {
 	for (size_t i = 0; i < g.size(); i++) {
 		if (g[i]->getDimension() == 2) {
 			OGRwkbGeometryType gt = OGR_GT_Flatten(g[i]->getGeometryType());
-			if (gt == wkbMultiSurface || gt == wkbMultiPolygon) {
+			if (OGR_GT_IsSubClassOf(gt, wkbGeometryCollection)) {
+				// will match OGRMultiPolygon, OGRMultiSurface and OGRGeometryCollection
 				OGRGeometryCollection *gc = (OGRGeometryCollection *) g[i];
 				out[i] = gc->get_Area();
-			} else {
+			} else if (OGR_GT_IsSurface(gt)) {
 				OGRSurface *surf = (OGRSurface *) g[i];
 				out[i] = surf->get_Area();
-			} 
+			} else {
+				out[i] = 0.0; // not supposed to happen, but who knows...
+			}
 		} else
 			out[i] = 0.0;
 		OGRGeometryFactory::destroyGeometry(g[i]);