Both Wayland and X11 backend are bound to the --windowed flag. Which
one to create is decided whether DISPLAY or WAYLAND_DISPLAY env variables
are set with Wayland having precedence over X11. Passing the display as
command line argument overwrites the env variable and takes further
precedence. E.g. if WAYLAND_DISPLAY is exported and --x11-display
argument is provided, it will create X11 backend. Similarly there is now
a --wayland-display command line argument.
If no backend got created, kwin_wayland will exit.
The singleton variant of WaylandBackend is adjusted to take the display
as argument and pass it to the Connection.
This reorders the startup sequence quite a bit:
1. Create QAbstractEventDispatcher and install it on QCoreApplication
2. Create Application
3. Start Xwayland, use thread to get when its ready
4. Create xcb connection
5. perform startup
For using the wayland QPA it needs a patch in QtWayland which will be
part of Qt 5.4.2, otherwise it blocks.
All of kwin except the main function goes into a new (private) library
called kwin. Two new kdeinit_executables are created:
* kwin_x11
* kwin_wayland
Both only use a dedicated main_x11.cpp and main_wayland.cpp with the
main function and a KWin::Application subclass and linking the new
kwin library.
The main idea behind this is to be able to perform more sane sanity
checks. E.g. on Wayland we don't need to first test whether we can
create an X11 connection. Instead we should abort if we cannot connect
to the Wayland display. Also the multi-head checks are not needed on
Wayland, etc. etc. As most of that code is in the main function to
simplify it's better to split.
This will also make it easier to diverge more easily in future. The
Wayland variant can introduce more suited command line arguments for
example. This already started by having the --replace option only
available in X11 variant. The Wayland backend is still a window manager,
but doesn't claim the manager selection.
