Use wlroots scene-graph API

Move xdg-shell and xwayland-shell surfaces to new API

Also render alt-tab on-screen-display by converting cairo-surface to
wlr_buffer

src/meson.build

@@ -16,10 +16,8 @@ labwc_sources = files(
   'seat.c',
   'server.c',
   'ssd.c',
-  'subsurface.c',
   'theme.c',
   'view.c',
-  'view-child.c',
   'view-impl.c',
   'xdg.c',
   'xdg-deco.c',