The target size is not always enough, there are cases where the offset
used to paint the target must also be available for developers
implementing an OffscreenEffect.
The get_target_rect() method returns the rectangle used to paint the
target, with the offsets in the ClutterRect:origin and the texture size
in the ClutterRect:size fields, respectively.
The get_target_size() method should be deprecated, given that its
replacement is generally more useful.
https://bugzilla.gnome.org/show_bug.cgi?id=670004
37e3ef2303
