wscreensaver: implement screensaver interface
wscreensaver: implement screensaver interface

Create instances from outputs, and register the surfaces as
screensavers. Support multiple "Mode" instances.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

Fantu/compositor-spice,eyolfson/weston,udoprog/weston,kwm81/weston,kwm81/weston,Tarnyko/weston-xdg_surface_present,udoprog/weston,Gnurou/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,eyolfson/weston,sir-murray/weston,xorgy/weston,jonnylamb/weston,sir-murray/weston,giucam/weston,krezovic/weston,mchalupa/weston,Gnurou/weston,xorgy/weston,jonnylamb/weston,mchalupa/weston,krezovic/weston,Fantu/compositor-spice,giucam/weston