Always draw the stage to an offscreen framebuffer when using the nested
backend, so that we more emulate things more similarly to how it works
real-world, i.e. it'll work the way whether stage views are enabled or
not.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
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