docs: Add example of setting background color with ClutterEffect

Add example of a simple background color effect applied via
pre_paint() implementation in a ClutterEffect subclass.

This is a simple effect with an incomplete GObject
implementation (no properties, setters or getters)
to make it as easy to follow as possible.

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

@@ -0,0 +1,42 @@
+#ifndef __CB_BACKGROUND_EFFECT_H__
+#define __CB_BACKGROUND_EFFECT_H__
+
+#include <clutter/clutter.h>
+
+GType cb_background_effect_get_type (void);
+
+#define CB_TYPE_BACKGROUND_EFFECT (cb_background_effect_get_type ())
+#define CB_BACKGROUND_EFFECT(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), \
+                                                               CB_TYPE_BACKGROUND_EFFECT, \
+                                                               CbBackgroundEffect))
+#define CB_IS_BACKGROUND_EFFECT(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), \
+                                                                          CB_TYPE_BACKGROUND_EFFECT))
+#define CB_BACKGROUND_EFFECT_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), \
+                                                                       CB_TYPE_BACKGROUND_EFFECT, \
+                                                                       CbBackgroundEffectClass))
+#define CB_IS_BACKGROUND_EFFECT_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), \
+                                                                       CB_TYPE_BACKGROUND_EFFECT))
+#define CB_BACKGROUND_EFFECT_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), \
+                                                                         CB_TYPE_BACKGROUND_EFFECT, \
+                                                                         CbBackgroundEffectClass))
+
+typedef struct _CbBackgroundEffectPrivate CbBackgroundEffectPrivate;
+typedef struct _CbBackgroundEffect        CbBackgroundEffect;
+typedef struct _CbBackgroundEffectClass   CbBackgroundEffectClass;
+
+/* object */
+struct _CbBackgroundEffect
+{
+  ClutterEffect              parent_instance;
+  CbBackgroundEffectPrivate *priv;
+};
+
+/* class */
+struct _CbBackgroundEffectClass
+{
+  ClutterEffectClass parent_class;
+};
+
+ClutterEffect *cb_background_effect_new ();
+
+#endif /* __CB_BACKGROUND_EFFECT_H__ */