Check boolean result manifoldness for shells individually. Fixes #516.

src/ifcgeom/IfcGeomFunctions.cpp

@@ -235,16 +235,26 @@ namespace {
 	}
 
 	bool is_manifold(const TopoDS_Shape& a) {
-		TopTools_IndexedDataMapOfShapeListOfShape map;
-		TopExp::MapShapesAndAncestors(a, TopAbs_EDGE, TopAbs_FACE, map);
-
-		for (int i = 1; i <= map.Extent(); ++i) {
-			if (map.FindFromIndex(i).Extent() != 2) {
-				return false;
+		if (a.ShapeType() == TopAbs_COMPOUND) {
+			TopoDS_Iterator it(a);
+			for (; it.More(); it.Next()) {
+				if (!is_manifold(it.Value())) {
+					return false;
+				}
 			}
-		}
+			return true;
+		} else {
+			TopTools_IndexedDataMapOfShapeListOfShape map;
+			TopExp::MapShapesAndAncestors(a, TopAbs_EDGE, TopAbs_FACE, map);
 
-		return true;
+			for (int i = 1; i <= map.Extent(); ++i) {
+				if (map.FindFromIndex(i).Extent() != 2) {
+					return false;
+				}
+			}
+
+			return true;
+		}
 	}
 
 	bool is_manifold(const TopTools_ListOfShape& l) {