- Adding "uint Gcalc_dyn_list::Item::m_item_id" with public method item_id().
  Item IDs exist only in DBUG mode.
  IDs are genereted consequently, starting from 1.
  With help of m_item_id it's much easier to identify items
  (e.g. res_point's and active_thread's) when debugging in gdb.
  Also, item_id() can be used DBUG_PRINT, to produce an easeir-to-read
  trace log.

- Encapsulation:
  Renaming res_point::outer_poly to "m_outer_poly".
  Moving it into the private section.
  Adding access methods: set_outer_poly() and get_outer_poly().
  This helps to set breakpoints when outer_poly is read or set.

- Moving setting of res_point::intersection_point and pes_point::pi
  inside Gcalc_operation_reducer::add_res_point(),
  to reduce code duplication.

- Introducing Gcalc_operation_reducer::add_res_i_point(),
  and moving setting of insersection_point, pi, x, y inside this method,
  to reduce code duplication.

- Introducing Gcalc_operation_reducer::add_res_single_point(),
  and moving setting of intesection_point, pi, x,y inside the method,
  to reduce code duplication.

- Moving new_res_point() from public to private section,
  as it's now used only inside the class any more.

- Moving DBUG_PRINT's inside add_res_point(), add_res_i_point(),
  and add_res_single_point(), to reduce code duplication.
  Adding printing of item_id(), to get an easeir-to-read trace log.