docs: Add example of a border added through ClutterEffect

Add a basic example showing how to implement a ClutterEffect
post_paint() function to overlay a highlight border over a
rectangular actor.

doc/cookbook/examples/cb-border-effect.h

@@ -0,0 +1,54 @@
+#ifndef __CB_BORDER_EFFECT_H__
+#define __CB_BORDER_EFFECT_H__
+
+#include <clutter/clutter.h>
+
+GType cb_border_effect_get_type (void);
+
+#define CB_TYPE_BORDER_EFFECT (cb_border_effect_get_type ())
+#define CB_BORDER_EFFECT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), \
+                                                           CB_TYPE_BORDER_EFFECT, \
+                                                           CbBorderEffect))
+#define CB_IS_BORDER_EFFECT(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), \
+                                                                      CB_TYPE_BORDER_EFFECT))
+#define CB_BORDER_EFFECT_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), \
+                                                                   CB_TYPE_BORDER_EFFECT, \
+                                                                   CbBorderEffectClass))
+#define CB_IS_BORDER_EFFECT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), \
+                                                                   CB_TYPE_BORDER_EFFECT))
+#define CB_BORDER_EFFECT_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), \
+                                                                     CB_TYPE_BORDER_EFFECT, \
+                                                                     CbBorderEffectClass))
+
+typedef struct _CbBorderEffectPrivate CbBorderEffectPrivate;
+typedef struct _CbBorderEffect        CbBorderEffect;
+typedef struct _CbBorderEffectClass   CbBorderEffectClass;
+
+/* object */
+struct _CbBorderEffect
+{
+  ClutterEffect          parent_instance;
+  CbBorderEffectPrivate *priv;
+};
+
+/* class */
+struct _CbBorderEffectClass
+{
+  ClutterEffectClass parent_class;
+};
+
+ClutterEffect *cb_border_effect_new (gfloat              width,
+                                     const ClutterColor *color);
+
+void cb_border_effect_set_color (CbBorderEffect     *self,
+                                 const ClutterColor *color);
+
+void cb_border_effect_get_color (CbBorderEffect *self,
+                                 ClutterColor   *color);
+
+void cb_border_effect_set_width (CbBorderEffect *self,
+                                 gfloat          width);
+
+gfloat cb_border_effect_get_width (CbBorderEffect *self);
+
+#endif /* __CB_BORDER_EFFECT_H__ */