Added override (in gnuplot file) for inside/outside path detection.

- cameo/cameo.c (process_paths): explain the inside/outside path detection
  heuristics
- cameo/path.h (struct path), cameo/path.c (path_new): added attribute
  "outside" to explicitly mark paths as outside edges
- cameo/path.c (path_from): new function to make a path that takes the
  attributes of another path
- cameo/path.c (path_reverse, path_offset): use path_from instead of
  path_new
- cameo/gnuplot.c (gnuplot_read, gnuplot_do_write): read and write the
  #%outside hint

cameo/path.h

@@ -23,6 +23,7 @@ struct point {
 struct path {
 	struct point *first, *last;
 	double r_tool;	/* mm */
+	int outside;	/* non-zero to mark path as an outside edge */
 	struct path *next;
 };