Add the screensaver interface to the desktop-shell protocol file. Also
add stubs for it in the compositor, and make wscreensaver to bind to the
screensaver interface. Wscreensaver gets a new option --demo to retain
the current behaviour as a regular wayland client.
When a screensaver application starts, it should bind to the screensaver
interface, enumerate all outputs, create a surface per output, and
register those surfaces via screensaver::set_surface request. Then it
continues with the usual animation loop, waiting for frame events. The
compositor will decide, when the given screensaver surfaces are
displayed. A screensaver application should respond to outputs coming
and going away by creating and destroying surfaces.
The compositor is supposed to activate a screensaver by exec'ing it, and
stop the screensaver by killing the client process. Only one client may
be bound to the screensaver interface at a time. If there already is a
client, the compositor could either kill it first, or not exec a new
one.
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
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