Base class for effects using offscreen buffers
<clutter-offscreen-effect> is an abstract class that can be used
by <clutter-effect> sub-classes requiring access to an offscreen
buffer.
Some effects, like the fragment shader based effects, can only use GL textures, and in order to apply those effects to any kind of actor they require that all drawing operations are applied to an offscreen framebuffer that gets redirected to a texture.
<clutter-offscreen-effect> provides all the heavy-lifting for
creating the offscreen framebuffer, the redirection and the final paint
of the texture on the desired stage.
Creating a sub-class of <clutter-offscreen-effect> requires, in
case of overriding the <clutter-effect> virtual functions, to
chain up to the <clutter-offscreen-effect>'s implementation.
On top of the <clutter-effect>'s virtual functions,
<clutter-offscreen-effect> also provides a
paint-target function, which encapsulates the effective
painting of the texture that contains the result of the offscreen
redirection.
The size of the target material is defined to be as big as the
transformed size of the <clutter-actor> using the offscreen
effect. Sub-classes of <clutter-offscreen-effect> can change the
texture creation code to provide bigger textures by overriding the
create-texture virtual function; no chain up to the
<clutter-offscreen-effect> implementation is required in this
case.
<clutter-offscreen-effect> is available since Clutter 1.4