The OffscreenEffect class needs to expose a way for sub-classes to
track the size of FBO it creates, in case it has to do some geometry
deformations like the DeformEffect sub-classes.
Let's move the private symbol we used internally in 1.6 to fix
DeformEffect to the list of public symbols of OffscreenEffect.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2570
Since the FBO target might have a different size than the mere paint box
of the actor, we need API to get it out of the ClutterOffscreenEffect
private data structure and on to sub-classes.
Since we cannot add new API in a stable cycle, we need a private
function; we'll leave it there even when opening 1.7, since it's useful
for internal purposes.