Bugfix for #954

The bug was that while iterating a Region during the gate traversal,
the "select_interacting" was triggering a sort() which changed the
order.

Solution is to pre-sort when iterators are issued also when the
iterator is non-region selecting. This way, plain and region query
iterators can be used together. In addition, the dirty flag scheme
of Cell+Shapes was cleaned up a little for bboxes.

src/db/unit_tests/dbShapeArrayTests.cc

@@ -119,7 +119,6 @@ TEST(2)
   shapes.insert (p2);
   shapes.insert (db::SimplePolygonRef (p2, *rep));
   shapes.sort ();
-  shapes.update_bbox ();
 
   EXPECT_EQ (shapes.bbox () == db::Box (100,-5200,2300,2000), true);