poly2d/: automatically switch between clockwise and counter-clockwise polygons

The only functions that care about this at the moment are from CGAL,
and there we can determine from context what to do.

poly2d/README

@@ -16,23 +16,22 @@ operation in cameo. Later, poly2d could replace more parts of
 cameo.
 
 poly2d puts more emphasis on simplicity than on performance. Some
-functions expect clockwise closed polygons that don't self-intersect.
+functions expect closed polygons that don't self-intersect. For now,
+it doesn't matter whether polygons are clockwise or counter-clockwise. 
 The table below shows the capabilities
 
-Open     Counter-clockwise
-|  Concave  Min. vertices
+Open     Min. vertices
+|  Concave
 |  |  Self-intersect
-|  |  |  |  |
-Y  Y  Y  Y  0		p2d_contains_poly(b), p2d_free, p2d_free_all,
-			p2d_is_closed, p2d_copy, p2d_reverse, p2d_vertices,
-			p2d_write_gnuplog, p2d_write_gnuplot_all
-Y  Y  Y  Y  1		p2d_read_gnuplot
--  Y  Y  Y  0		p2d_simplify
--  Y  -  Y  3		p2d_is_cw
--  Y  -  -  3		p2d_contains_point, p2d_contains_poly (a),
--  Y  -  #  3		p2d_area*, p2d_offset*
-
-# CGAL uses ccw, poly2d uses cw. Need to switch.
+|  |  |  |
+Y  Y  Y  0		p2d_contains_poly(b), p2d_copy, p2d_free, p2d_free_all,
+			p2d_is_closed, p2d_no_intersect, p2d_reverse,
+			p2d_vertices, p2d_write_gnuplot, p2d_write_gnuplot_all
+Y  Y  Y  1		p2d_read_gnuplot
+-  Y  Y  0		p2d_simplify
+-  Y  -  3		p2d_is_cw
+-  Y  -  3		p2d_contains_point, p2d_contains_poly (a),
+-  Y  -  3		p2d_area*, p2d_offset*
 
 Not yet implemented:
 - p2d_simplify (low priority - the offsetting from CGAL already covers
@@ -43,8 +42,6 @@ Not yet specified:
 
 Other:
 - change the license from GPL to LGPL
-- change ccw to cw
-- make sure CGAL is only fed ccw ploygons and cw holes
 - transform CGAL's idea of outer polygons into something we can use
 - use more meaningful offset/overlap model for area fill
 - check for memory leaks