color-cycle: add color generator helper

We need a simple helper to choose colors for visualizers so
they do not overlap. This is just a simple cycle using some
predefined colors.

We should come up with something more unique in the future or
possibly do this with CSS and :nth-child(n) once we have access
to CSS nodes.

lib/sp-color-cycle.h

@@ -0,0 +1,39 @@
+/* sp-color-cycle.h
+ *
+ * Copyright (C) 2016 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef SP_COLOR_CYCLE_H
+#define SP_COLOR_CYCLE_H
+
+#include <gtk/gtk.h>
+
+G_BEGIN_DECLS
+
+#define SP_TYPE_COLOR_CYCLE (sp_color_cycle_get_type())
+
+typedef struct _SpColorCycle SpColorCycle;
+
+GType         sp_color_cycle_get_type (void);
+SpColorCycle *sp_color_cycle_ref      (SpColorCycle *self);
+void          sp_color_cycle_unref    (SpColorCycle *self);
+SpColorCycle *sp_color_cycle_new      (void);
+void          sp_color_cycle_next     (SpColorCycle *self,
+                                       GdkRGBA      *rgba);
+
+G_END_DECLS
+
+#endif /* SP_COLOR_CYCLE_H */