Split up the MetaRendererX11 class into one for when running as a
X11 compositing manager, and one for when running as a nested Wayland
compositor.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
MetaRenderer is meant to be the object responsible for rendering the
scene graph. It will contain the logic related to the cogl winsys
backend, the clutter backend, and the clutter stage window.
https://bugzilla.gnome.org/show_bug.cgi?id=768976