Revision 686

Safer implementation of the performance enhancements of the
low-level renderer:

* extensions/thuban/wxproj.cpp (extract_projection)
(extract_pointer): Rename extract_projection to extract_pointer
and redefine its purpose to return the pointer stored in a CObject
returned by the object's cobject method. Update all callers.
(s_draw_info, free_draw_info, draw_polygon_init): Implement the
handling of these low-level parameters so that each s_draw_info
instance is handled as a CObject at python level that also
contains real references to the actual python objects which
contain the values in the struct. Add free_draw_info as the
destructor.
(draw_polygon_shape): Add the py_draw_info parameter which must a
cobject containing an s_draw_info pointer.

* Thuban/UI/renderer.py (MapRenderer.polygon_render_param): New
method to instantiat the low-level render parameter
(MapRenderer.draw_shape_layer): Use the new method. Remove some
commented out code.
(MapRenderer.draw_polygon_shape): Make the first parameter not the
layer but the low-level render parameter
(ScreenRenderer.draw_shape_layer): Use the low-level render
parameter.