Sometimes a subclass of ClutterOffscreenEffect wants to paint with a
completely custom material. In that case it is awkward to modify the
material returned owned by ClutterOffscreenEffect so it makes more
sense to just get the texture and manage its own material.
offscreen-effect: Track the size of the actor separately
Previously the offscreen effect was keeping track of the size of the
texture so that it could detect when a different size is requested and
create a new texture. However this breaks if a subclass overrides
create_texture to make the texture bigger because in that case the
size of the texture will always be different from the calculated size
of the actor. This patch makes it also track the size of the fbo that
was requested before being passed through create_texture() and it
instead uses that to detect when a new FBO is needed.
