Unfortunately, many parts of GNOME Shell and Mutter and Clutter
still use the implicit Cogl1 API. As such, it as a transition
between the old and new APIs, it is important to keep the
implicit draw framebuffer updated.
ClutterRootNode does not keep it updated though, and it might
lead to problems when rendering offscreen textures.
Fix that by pushing and popping the root node framebuffer on
pre- and post-draw, respectively.