pre_paint() and post_paint() implementations don't need
to check whether an effect is disabled: Clutter will
not apply an effect unless it is enabled.
So remove code which checks whether the effect is
enabled or disabled from the example applications and the
documentation.
Add a recipe showing how to implement two simple
effects, based on ClutterEffect: an always gray background,
and a border with configurable width and color.
Also explains the necessity to queue a redraw on
the associated actor if the effect's properties change,
and shows how to implement that.
The example gives the GObject code for both effects,
as well as an example application showing how to use them.
The example also demonstrates how to disable/enable an effect,
making the border round an actor togglable.
Add an effects chapter which gives a broad overview of
the abstract classes in the effects API, plus a short
example of how to apply one of the stock Clutter
effects (ClutterColorizeEffect).
The recipe explains how to create a custom ClutterDeformEffect
to produce a page fold (code based on ClutterPageTurnEffect).
The example code includes the effect class plus a small
application to apply it to a texture.
