Terminate on parent compositor exit
Test plan: open rooston, and open another rootston inside it (but not from within rootston):
- With the Wayland backend:
env WAYLAND_DISPLAY=wayland-1 build/rootston/rootston
- With the X11 backend:
env WAYLAND_DISPLAY= DISPLAY=:1 build/rootston/rootston
Kill the outer rootston. Check that the inner one exits.
Fixes #397 (closed)