wayland: Make display closing work
wayland: Make display closing work

The code in _gdk_wayland_window_dispose was not safe against
being called twice - it would call g_hash_table_destroy twice
on the known_globals hash table, the second time operating on
freed memory. It was also leaking the list of async_roundtrips.

After fixing both of these issues, the displayclose testcase
now works on Wayland.

davidgumberg/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,jigpu/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,chergert/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,chergert/gtk