The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.
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
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
