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/cameo.c

@@ -37,10 +37,24 @@ static void process_paths(struct path *paths)
 {
 	struct path *leftmost, *path;
 
+	/*
+	 * We don't have an algorithm (yet) that can detect which paths are
+	 * inside other paths. Therefore, we fake it by looking for the path
+	 * that contains lowest x coordinate. This ought to be the outer
+	 * boundary of the piece.
+	 *
+	 * Note that this heuristic falls apart when a job consists of
+	 * multiple pieces. In this case, the #%outside hint can be used to
+	 * explicitly tell cameo to treat the path as an outside edge.
+	 */
+
 	leftmost = path_find_leftmost(paths);
 	for (path = paths; path; path = path->next)
-		if (path != leftmost)
+		if (path != leftmost && !path->outside)
 			process_path(path, 1);
+	for (path = paths; path; path = path->next)
+		if (path != leftmost && path->outside)
+			process_path(path, 0);
 	process_path(leftmost, 0);
 }