xwayland: Fix infinite loop at startup
Mutter recently added headless tests, and when running those tests the Wayland compositor runs for a very short time.
Xwayland is spawned by the Wayland compositor and upon startup will query the various Wayland protocol supported by the compositor.
To do so, it will do a roundtrip to the Wayland server waiting for events it expects.
If the Wayland compositor terminates before Xwayland has got the replies
it expects, it will loop indefinitely calling wl_display_roundtrip()
continuously.
To avoid that issue, add a new xwl_screen_roundtrip() that checks for
the returned value from wl_display_roundtrip() and fails if it is
negative.
Signed-off-by: Olivier Fourdan ofourdan@redhat.com