Split the stage window implementations into three separate objects: one
for X11 as a compositing manager, one for X11 running as a nested
Wayland compositor, and one for running with the native backend.
The new stage window implementations are only thin shells; this is in
preparation for making the stage windows behave more differently.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
Instead of passing around the KMS file descriptor via clutter to cogl,
just make our own clutter backend create the cogl renderer and set the
KSM fd.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
In cogl use cogl-config.h and in clutter use clutter-build-config.h. We
can't use clutter-config.h in clutter because its already used and
installed.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
Introduce two new clutter backends: MetaClutterBackendX11 and
MetaClutterBackendNative. They are so far only wrap ClutterBackendX11
and ClutterBackendEglNative respectively, but the aim is to move things
from the original clutter backends when needed.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
